diff --git a/src/include/libpq/pqcomm.h b/src/include/libpq/pqcomm.h
index bff7dd18a2..8251639cd3 100644
--- a/src/include/libpq/pqcomm.h
+++ b/src/include/libpq/pqcomm.h
@@ -111,8 +111,8 @@ extern PGDLLIMPORT bool Db_user_namespace;
 /* These are the authentication request codes sent by the backend. */
 
 #define AUTH_REQ_OK			0	/* User is authenticated  */
-#define AUTH_REQ_KRB4		1	/* Kerberos V4. Not supported any more. */
-#define AUTH_REQ_KRB5		2	/* Kerberos V5. Not supported any more. */
+/* 1 is available. It was used for Kerberos V4, not supported any more  */
+/* 2 is available. It was used for Kerberos V5, not supported any more  */
 #define AUTH_REQ_PASSWORD	3	/* Password */
 #define AUTH_REQ_CRYPT		4	/* crypt password. Not supported any more. */
 #define AUTH_REQ_MD5		5	/* md5 password */
diff --git a/src/interfaces/libpq/fe-auth.c b/src/interfaces/libpq/fe-auth.c
index fa95f8e6e9..1ce0794f89 100644
--- a/src/interfaces/libpq/fe-auth.c
+++ b/src/interfaces/libpq/fe-auth.c
@@ -942,14 +942,6 @@ pg_fe_sendauth(AuthRequest areq, int payloadlen, PGconn *conn)
 		case AUTH_REQ_OK:
 			break;
 
-		case AUTH_REQ_KRB4:
-			libpq_append_conn_error(conn, "Kerberos 4 authentication not supported");
-			return STATUS_ERROR;
-
-		case AUTH_REQ_KRB5:
-			libpq_append_conn_error(conn, "Kerberos 5 authentication not supported");
-			return STATUS_ERROR;
-
 #if defined(ENABLE_GSS) || defined(ENABLE_SSPI)
 		case AUTH_REQ_GSS:
 #if !defined(ENABLE_SSPI)
diff --git a/doc/src/sgml/protocol.sgml b/doc/src/sgml/protocol.sgml
index 8b5e7b1ad7..ed7a3ceda1 100644
--- a/doc/src/sgml/protocol.sgml
+++ b/doc/src/sgml/protocol.sgml
@@ -271,20 +271,6 @@
       </listitem>
      </varlistentry>
 
-     <varlistentry>
-      <term>AuthenticationKerberosV5</term>
-      <listitem>
-       <para>
-        The frontend must now take part in a Kerberos V5
-        authentication dialog (not described here, part of the
-        Kerberos specification) with the server.  If this is
-        successful, the server responds with an AuthenticationOk,
-        otherwise it responds with an ErrorResponse. This is no
-        longer supported.
-       </para>
-      </listitem>
-     </varlistentry>
-
      <varlistentry>
       <term>AuthenticationCleartextPassword</term>
       <listitem>
@@ -3318,41 +3304,6 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
     </listitem>
    </varlistentry>
 
-   <varlistentry id="protocol-message-formats-AuthenticationKerberosV5">
-    <term>AuthenticationKerberosV5 (B)</term>
-    <listitem>
-
-     <variablelist>
-      <varlistentry>
-       <term>Byte1('R')</term>
-       <listitem>
-        <para>
-         Identifies the message as an authentication request.
-        </para>
-       </listitem>
-      </varlistentry>
-
-      <varlistentry>
-       <term>Int32(8)</term>
-       <listitem>
-        <para>
-         Length of message contents in bytes, including self.
-        </para>
-       </listitem>
-      </varlistentry>
-
-      <varlistentry>
-       <term>Int32(2)</term>
-       <listitem>
-        <para>
-         Specifies that Kerberos V5 authentication is required.
-        </para>
-       </listitem>
-      </varlistentry>
-     </variablelist>
-    </listitem>
-   </varlistentry>
-
    <varlistentry id="protocol-message-formats-AuthenticationCleartextPassword">
     <term>AuthenticationCleartextPassword (B)</term>
     <listitem>