diff --git a/Lib/pickle.py b/Lib/pickle.py
index 2947bd4..409d4b2 100644
--- a/Lib/pickle.py
+++ b/Lib/pickle.py
@@ -470,7 +470,7 @@
             else:
                 self.write(LONG4 + pack("<i", n) + encoded)
             return
-        self.write(LONG + repr(obj).encode("ascii") + b'\n')
+        self.write(LONG + repr(obj).encode("ascii") + b'L\n')
     dispatch[int] = save_long
 
     def save_float(self, obj, pack=struct.pack):
@@ -890,6 +890,8 @@
 
     def load_long(self):
         val = self.readline()[:-1].decode("ascii")
+        if val and val[-1] == 'L':
+            val = val[:-1]
         self.append(int(val, 0))
     dispatch[LONG[0]] = load_long
 
diff --git a/Lib/pickletools.py b/Lib/pickletools.py
index 1dd45da..6614df7 100644
--- a/Lib/pickletools.py
+++ b/Lib/pickletools.py
@@ -527,6 +527,8 @@
     """
 
     s = read_stringnl(f, decode=False, stripquotes=False)
+    if s[-1:] == b'L':
+        s = s[:-1]
     return int(s)
 
 
@@ -2052,39 +2054,39 @@
     1: l        LIST       (MARK at 0)
     2: p    PUT        0
     5: L    LONG       1
-    8: a    APPEND
-    9: L    LONG       2
-   12: a    APPEND
-   13: (    MARK
-   14: L        LONG       3
-   17: L        LONG       4
-   20: t        TUPLE      (MARK at 13)
-   21: p    PUT        1
-   24: a    APPEND
-   25: (    MARK
-   26: d        DICT       (MARK at 25)
-   27: p    PUT        2
-   30: c    GLOBAL     'builtins bytes'
-   46: p    PUT        3
-   49: (    MARK
-   50: (        MARK
-   51: l            LIST       (MARK at 50)
-   52: p        PUT        4
-   55: L        LONG       97
-   59: a        APPEND
-   60: L        LONG       98
+    9: a    APPEND
+   10: L    LONG       2
+   14: a    APPEND
+   15: (    MARK
+   16: L        LONG       3
+   20: L        LONG       4
+   24: t        TUPLE      (MARK at 15)
+   25: p    PUT        1
+   28: a    APPEND
+   29: (    MARK
+   30: d        DICT       (MARK at 29)
+   31: p    PUT        2
+   34: c    GLOBAL     'builtins bytes'
+   50: p    PUT        3
+   53: (    MARK
+   54: (        MARK
+   55: l            LIST       (MARK at 54)
+   56: p        PUT        4
+   59: L        LONG       97
    64: a        APPEND
-   65: L        LONG       99
-   69: a        APPEND
-   70: t        TUPLE      (MARK at 49)
-   71: p    PUT        5
-   74: R    REDUCE
-   75: p    PUT        6
-   78: V    UNICODE    'def'
-   83: p    PUT        7
-   86: s    SETITEM
-   87: a    APPEND
-   88: .    STOP
+   65: L        LONG       98
+   70: a        APPEND
+   71: L        LONG       99
+   76: a        APPEND
+   77: t        TUPLE      (MARK at 53)
+   78: p    PUT        5
+   81: R    REDUCE
+   82: p    PUT        6
+   85: V    UNICODE    'def'
+   90: p    PUT        7
+   93: s    SETITEM
+   94: a    APPEND
+   95: .    STOP
 highest protocol among opcodes = 0
 
 Try again with a "binary" pickle.
@@ -2157,12 +2159,12 @@
    92: V    UNICODE    'value'
    99: p    PUT        7
   102: L    LONG       42
-  106: s    SETITEM
-  107: b    BUILD
-  108: a    APPEND
-  109: g    GET        5
-  112: a    APPEND
-  113: .    STOP
+  107: s    SETITEM
+  108: b    BUILD
+  109: a    APPEND
+  110: g    GET        5
+  113: a    APPEND
+  114: .    STOP
 highest protocol among opcodes = 0
 
 >>> dis(pickle.dumps(x, 1))
diff --git a/Lib/test/pickletester.py b/Lib/test/pickletester.py
index 887b512..3ed84fb 100644
--- a/Lib/test/pickletester.py
+++ b/Lib/test/pickletester.py
@@ -90,21 +90,21 @@
 # the object returned by create_data().
 
 DATA0 = (
-    b'(lp0\nL0\naL1\naF2.0\nac'
+    b'(lp0\nL0L\naL1L\naF2.0\nac'
     b'builtins\ncomplex\n'
     b'p1\n(F3.0\nF0.0\ntp2\nRp'
-    b'3\naL1\naL-1\naL255\naL-'
-    b'255\naL-256\naL65535\na'
-    b'L-65535\naL-65536\naL2'
-    b'147483647\naL-2147483'
-    b'647\naL-2147483648\na('
+    b'3\naL1L\naL-1L\naL255L\naL-'
+    b'255L\naL-256L\naL65535L\na'
+    b'L-65535L\naL-65536L\naL2'
+    b'147483647L\naL-2147483'
+    b'647L\naL-2147483648L\na('
     b'Vabc\np4\ng4\nccopyreg'
     b'\n_reconstructor\np5\n('
     b'c__main__\nC\np6\ncbu'
     b'iltins\nobject\np7\nNt'
     b'p8\nRp9\n(dp10\nVfoo\np1'
-    b'1\nL1\nsVbar\np12\nL2\nsb'
-    b'g9\ntp13\nag13\naL5\na.'
+    b'1\nL1L\nsVbar\np12\nL2L\nsb'
+    b'g9\ntp13\nag13\naL5L\na.'
 )
 
 # Disassembly of DATA0
@@ -113,80 +113,80 @@
     1: l        LIST       (MARK at 0)
     2: p    PUT        0
     5: L    LONG       0
-    8: a    APPEND
-    9: L    LONG       1
-   12: a    APPEND
-   13: F    FLOAT      2.0
-   18: a    APPEND
-   19: c    GLOBAL     'builtins complex'
-   37: p    PUT        1
-   40: (    MARK
-   41: F        FLOAT      3.0
-   46: F        FLOAT      0.0
-   51: t        TUPLE      (MARK at 40)
-   52: p    PUT        2
-   55: R    REDUCE
-   56: p    PUT        3
-   59: a    APPEND
-   60: L    LONG       1
-   63: a    APPEND
-   64: L    LONG       -1
-   68: a    APPEND
-   69: L    LONG       255
-   74: a    APPEND
-   75: L    LONG       -255
-   81: a    APPEND
-   82: L    LONG       -256
-   88: a    APPEND
-   89: L    LONG       65535
-   96: a    APPEND
-   97: L    LONG       -65535
-  105: a    APPEND
-  106: L    LONG       -65536
+    9: a    APPEND
+   10: L    LONG       1
+   14: a    APPEND
+   15: F    FLOAT      2.0
+   20: a    APPEND
+   21: c    GLOBAL     'builtins complex'
+   39: p    PUT        1
+   42: (    MARK
+   43: F        FLOAT      3.0
+   48: F        FLOAT      0.0
+   53: t        TUPLE      (MARK at 42)
+   54: p    PUT        2
+   57: R    REDUCE
+   58: p    PUT        3
+   61: a    APPEND
+   62: L    LONG       1
+   66: a    APPEND
+   67: L    LONG       -1
+   72: a    APPEND
+   73: L    LONG       255
+   79: a    APPEND
+   80: L    LONG       -255
+   87: a    APPEND
+   88: L    LONG       -256
+   95: a    APPEND
+   96: L    LONG       65535
+  104: a    APPEND
+  105: L    LONG       -65535
   114: a    APPEND
-  115: L    LONG       2147483647
-  127: a    APPEND
-  128: L    LONG       -2147483647
-  141: a    APPEND
-  142: L    LONG       -2147483648
-  155: a    APPEND
-  156: (    MARK
-  157: V        UNICODE    'abc'
-  162: p        PUT        4
-  165: g        GET        4
-  168: c        GLOBAL     'copyreg _reconstructor'
-  192: p        PUT        5
-  195: (        MARK
-  196: c            GLOBAL     '__main__ C'
-  208: p            PUT        6
-  211: c            GLOBAL     'builtins object'
-  228: p            PUT        7
-  231: N            NONE
-  232: t            TUPLE      (MARK at 195)
-  233: p        PUT        8
-  236: R        REDUCE
-  237: p        PUT        9
-  240: (        MARK
-  241: d            DICT       (MARK at 240)
-  242: p        PUT        10
-  246: V        UNICODE    'foo'
-  251: p        PUT        11
-  255: L        LONG       1
-  258: s        SETITEM
-  259: V        UNICODE    'bar'
-  264: p        PUT        12
-  268: L        LONG       2
-  271: s        SETITEM
-  272: b        BUILD
-  273: g        GET        9
-  276: t        TUPLE      (MARK at 156)
-  277: p    PUT        13
-  281: a    APPEND
-  282: g    GET        13
-  286: a    APPEND
-  287: L    LONG       5
-  290: a    APPEND
-  291: .    STOP
+  115: L    LONG       -65536
+  124: a    APPEND
+  125: L    LONG       2147483647
+  138: a    APPEND
+  139: L    LONG       -2147483647
+  153: a    APPEND
+  154: L    LONG       -2147483648
+  168: a    APPEND
+  169: (    MARK
+  170: V        UNICODE    'abc'
+  175: p        PUT        4
+  178: g        GET        4
+  181: c        GLOBAL     'copyreg _reconstructor'
+  205: p        PUT        5
+  208: (        MARK
+  209: c            GLOBAL     '__main__ C'
+  221: p            PUT        6
+  224: c            GLOBAL     'builtins object'
+  241: p            PUT        7
+  244: N            NONE
+  245: t            TUPLE      (MARK at 208)
+  246: p        PUT        8
+  249: R        REDUCE
+  250: p        PUT        9
+  253: (        MARK
+  254: d            DICT       (MARK at 253)
+  255: p        PUT        10
+  259: V        UNICODE    'foo'
+  264: p        PUT        11
+  268: L        LONG       1
+  272: s        SETITEM
+  273: V        UNICODE    'bar'
+  278: p        PUT        12
+  282: L        LONG       2
+  286: s        SETITEM
+  287: b        BUILD
+  288: g        GET        9
+  291: t        TUPLE      (MARK at 169)
+  292: p    PUT        13
+  296: a    APPEND
+  297: g    GET        13
+  301: a    APPEND
+  302: L    LONG       5
+  306: a    APPEND
+  307: .    STOP
 highest protocol among opcodes = 0
 """
 
diff --git a/Misc/NEWS b/Misc/NEWS
index 1050ae4..d20d20b 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -134,6 +134,9 @@
 Library
 -------
 
+- Issue #4842: Always append a trailing 'L' when pickling longs using
+  pickle protocol 0.  When reading, the 'L' is optional.
+
 - Add the importlib package.
 
 - Issue #4301: Patch the logging module to add processName support, remove
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index 6cc90b3..02a3e44 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -846,8 +846,8 @@
         /* Text-mode pickle, or long too big to fit in the 4-byte
          * signed BININT format:  store as a string.
          */
-        pdata[0] = LONG;        /* use LONG for consistence with pickle.py */
-        PyOS_snprintf(pdata + 1, sizeof(pdata) - 1, "%ld\n", x);
+        pdata[0] = LONG;        /* use LONG for consistency with pickle.py */
+        PyOS_snprintf(pdata + 1, sizeof(pdata) - 1, "%ldL\n", x);
         if (pickler_write(self, pdata, strlen(pdata)) < 0)
             return -1;
     }
@@ -977,8 +977,9 @@
     else {
         char *string;
 
-        /* proto < 2:  write the repr and newline.  This is quadratic-time
-           (in the number of digits), in both directions. */
+        /* proto < 2: write the repr and newline.  This is quadratic-time (in
+           the number of digits), in both directions.  We add a trailing 'L'
+           to the repr, for compatibility with Python 2.x. */
 
         repr = PyObject_Repr(obj);
         if (repr == NULL)
@@ -990,7 +991,7 @@
 
         if (pickler_write(self, &long_op, 1) < 0 ||
             pickler_write(self, string, size) < 0 ||
-            pickler_write(self, "\n", 1) < 0)
+            pickler_write(self, "L\n", 2) < 0)
             goto error;
     }
 
@@ -2880,7 +2881,7 @@
 load_long(UnpicklerObject *self)
 {
     PyObject *value;
-    char *s;
+    char *s, *ss;
     Py_ssize_t len;
 
     if ((len = unpickler_readline(self, &s)) < 0)
@@ -2888,8 +2889,27 @@
     if (len < 2)
         return bad_readline();
 
-    /* XXX: Should the base argument explicitly set to 10? */
-    if ((value = PyLong_FromString(s, NULL, 0)) == NULL)
+    /* s[len-2] will usually be 'L' (and s[len-1] is '\n'); we need to remove
+       the 'L' before calling PyLong_FromString.  In order to maintain
+       compatibility with Python 3.0.0, we don't actually *require*
+       the 'L' to be present. */
+    if (s[len-2] == 'L') {
+        ss = (char *)PyMem_Malloc(len-1);
+        if (ss == NULL) {
+            PyErr_NoMemory();
+            return -1;
+        }
+        strncpy(ss, s, len-2);
+        ss[len-2] = '\0';
+
+        /* XXX: Should the base argument explicitly set to 10? */
+        value = PyLong_FromString(ss, NULL, 0);
+        PyMem_Free(ss);
+    }
+    else {
+        value = PyLong_FromString(s, NULL, 0);
+    }
+    if (value == NULL)
         return -1;
 
     PDATA_PUSH(self->stack, value, -1);