diff -r 3ed5a6030c9b Lib/imaplib.py
--- a/Lib/imaplib.py	Mon Jan 02 19:18:02 2012 +0100
+++ b/Lib/imaplib.py	Tue Jan 03 22:31:43 2012 +0200
@@ -1295,7 +1295,7 @@
         #  so when it gets to the end of the 8-bit input
         #  there's no partial 6-bit output.
         #
-        oup = ''
+        oup = b''
         while inp:
             if len(inp) > 48:
                 t = inp[:48]
@@ -1303,7 +1303,7 @@
             else:
                 t = inp
                 inp = ''
-            e = binascii.b2a_base64(t)
+            e = binascii.b2a_base64(t.encode())
             if e:
                 oup = oup + e[:-1]
         return oup
diff -r 3ed5a6030c9b Lib/test/test_imaplib.py
--- a/Lib/test/test_imaplib.py	Mon Jan 02 19:18:02 2012 +0100
+++ b/Lib/test/test_imaplib.py	Tue Jan 03 22:31:43 2012 +0200
@@ -72,6 +72,9 @@
 
     timeout = 1
 
+    # Function to handle continuation input
+    _contcmd = None
+
     def _send(self, message):
         if verbose: print("SENT: %r" % message.strip())
         self.wfile.write(message)
@@ -98,6 +101,10 @@
                     break
 
             if verbose: print('GOT: %r' % line.strip())
+            if self._contcmd is not None:
+                self._contcmd(tag, line)
+                continue
+
             splitline = line.split()
             tag = splitline[0].decode('ASCII')
             cmd = splitline[1].decode('ASCII')
@@ -161,6 +168,50 @@
             self.reap_server(server, thread)
 
     @reap_threads
+    def test_authenticate_plain(self):
+        """ Tests PLAIN authentication and logging out """
+
+        class AuthHandler(SimpleIMAPHandler):
+
+            def cmd_CAPABILITY(self, tag, args):
+                self._send(b'* CAPABILITY IMAP4rev1 LOGINDISABLED AUTH=PLAIN\r\n')
+                self._send('{} OK Capability completed.\r\n'.format(tag).encode('ASCII'))
+
+            def cmd_AUTHENTICATE(self, tag, args):
+                self._send(b'+ \r\n')
+                self._contcmd = self._cmd_AUTHENTICATE_CONT
+
+            def _cmd_AUTHENTICATE_CONT(self, tag, args):
+                if args == b"dXNlcgB1c2VyAHBhc3M=\r\n":
+                    self._send('{} OK PLAIN authentication successful\r\n'
+                               .format(tag).encode('ASCII'))
+                else:
+                    self._send('{} NO No access\r\n'.format(tag).encode('ASCII'))
+                self._contcmd = None
+
+            def cmd_LOGOUT(self, tag, args):
+                self._send(b'* BYE Logging out\r\n')
+                self._send('{} OK Logout completed.\r\n'.format(tag).encode('ASCII'))
+
+        with self.reaped_server(AuthHandler) as server:
+            try:
+                client = self.imap_class(*server.server_address)
+            except imaplib.IMAP4.abort:
+                self.fail("IMAP4 connection failed")
+
+            self.assertTrue('AUTH=PLAIN' in client.capabilities)
+
+            try:
+                authobj = lambda resp: "{0}\x00{0}\x00{1}".format("user", "pass")
+                ret, data = client.authenticate("PLAIN", authobj)
+                self.assertEqual(ret, "OK")
+            except imaplib.error:
+                self.fail("PLAIN authentication failed")
+
+            ret, data = client.logout()
+            self.assertEqual(ret, "BYE")
+
+    @reap_threads
     def test_connect(self):
         with self.reaped_server(SimpleIMAPHandler) as server:
             client = self.imap_class(*server.server_address)