diff --git a/lib/net/smtp.rb b/lib/net/smtp.rb
index d06eda2..f774381 100644
--- a/lib/net/smtp.rb
+++ b/lib/net/smtp.rb
@@ -840,6 +840,9 @@ def open_message_stream(from_addr, *to_addrs, &block)   # :yield: stream
     DEFAULT_AUTH_TYPE = :plain
 
     # call-seq:
+    #   authenticate(type: DEFAULT_AUTH_TYPE, **, &)
+    #   authenticate(type = DEFAULT_AUTH_TYPE, **, &)
+    #   authenticate(username, secret, type: DEFAULT_AUTH_TYPE, **, &)
     #   authenticate(username, secret, type = DEFAULT_AUTH_TYPE, **, &)
     #
     # Authenticates with the server, using the "AUTH" command.
@@ -849,11 +852,17 @@ def open_message_stream(from_addr, *to_addrs, &block)   # :yield: stream
     # All arguments—other than +type+—are forwarded to the authenticator.
     # Different authenticators may interpret the +username+ and +secret+
     # arguments differently.
-    def authenticate(user, secret, authtype = DEFAULT_AUTH_TYPE)
-      check_auth_method authtype
-      check_auth_args user, secret
-      authenticator = Authenticator.auth_class(authtype).new(self)
-      critical { authenticator.auth(user, secret) }
+    #
+    # #authenticate always sends either two positional arguments to the
+    # authenticator or none.  Use #auth to send a different number of arguments.
+    def authenticate(*args, **kwargs, &block)
+      case args.length
+      when 1, 3 then authtype = args.pop
+      when (4..)
+        raise ArgumentError, "wrong number of arguments " \
+                             "(given %d, expected 0..3)" % [args.length]
+      end
+      auth(authtype, *args, **kwargs, &block)
     end
 
     # call-seq: