diff --git a/Lib/test/pickletester.py b/Lib/test/pickletester.py
--- a/Lib/test/pickletester.py
+++ b/Lib/test/pickletester.py
@@ -3,6 +3,7 @@
 import pickle
 import pickletools
 import random
+import struct
 import sys
 import unittest
 import weakref
@@ -1633,8 +1634,10 @@
                         continue
                     try:
                         pickled = self.dumps(data, protocol=proto)
-                        self.assertTrue(b"abcd" in pickled[:19])
-                        self.assertTrue(b"abcd" in pickled[-18:])
+                        header = pickle.BINBYTES
+                        header += struct.pack("<I", len(data))
+                        self.assertTrue(header in pickled)
+                        self.assertTrue(data in pickled)
                     finally:
                         pickled = None
         finally:
@@ -1648,8 +1651,20 @@
                 with self.subTest(proto=proto):
                     if proto < 3:
                         continue
-                    with self.assertRaises((ValueError, OverflowError)):
-                        self.dumps(data, protocol=proto)
+                    elif proto == 3:
+                        # Protocol 3 does not support large bytes objects.
+                        # Verify that we do not crash when processing one.
+                        with self.assertRaises((ValueError, OverflowError)):
+                            self.dumps(data, protocol=proto)
+                        continue
+                    try:
+                        pickled = self.dumps(data, protocol=proto)
+                        header = pickle.BINBYTES8
+                        header += struct.pack("<Q", len(data))
+                        self.assertTrue(header in pickled)
+                        self.assertTrue(data in pickled)
+                    finally:
+                        pickled = None
         finally:
             data = None
 
@@ -1664,8 +1679,14 @@
                 with self.subTest(proto=proto):
                     try:
                         pickled = self.dumps(data, protocol=proto)
-                        self.assertTrue(b"abcd" in pickled[:19])
-                        self.assertTrue(b"abcd" in pickled[-18:])
+                        if proto == 0:
+                            header = pickle.UNICODE
+                        else:
+                            header = pickle.BINUNICODE
+                            header += struct.pack("<I", len(data))
+                        self.assertTrue(header in pickled)
+                        self.assertEqual((pickled.rindex(b"abcd") + 4 -
+                                          pickled.index(b"abcd")), len(data))
                     finally:
                         pickled = None
         finally:
@@ -1686,13 +1707,16 @@
                     if proto < 4:
                         with self.assertRaises((ValueError, OverflowError)):
                             self.dumps(data, protocol=proto)
-                    else:
-                        try:
-                            pickled = self.dumps(data, protocol=proto)
-                            self.assertTrue(b"abcd" in pickled[:19])
-                            self.assertTrue(b"abcd" in pickled[-18:])
-                        finally:
-                            pickled = None
+                        continue
+                    try:
+                        pickled = self.dumps(data, protocol=proto)
+                        header = pickle.BINUNICODE8
+                        header += struct.pack("<Q", len(data))
+                        self.assertTrue(header in pickled)
+                        self.assertEqual((pickled.rindex(b"abcd") + 4 -
+                                          pickled.index(b"abcd")), len(data))
+                    finally:
+                        pickled = None
         finally:
             data = None
 
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -2027,7 +2027,7 @@
         else if (self->proto >= 4) {
             header[0] = BINBYTES8;
             _write_size64(header + 1, size);
-            len = 8;
+            len = 9;
         }
         else {
             PyErr_SetString(PyExc_OverflowError,