Abstract: A plumbing fitting comprising a test closure plug for the free end of a trap bushing mounted in the center port of a T-fitting constituting a part of a soil pipe and vent pipe assembly of a plumbing system the free end of the trap bushing is provided with an adapter having an adapter nut and washer. The washer is removed and the plug is inserted in the adapter opening. The plug is configured to close the adapter opening and make a seal therewith when held in place by the adapter nut. The plug enables pressure testing of the plumbing system or the maintenance of a head of water therein. Upon installation of a fixture with a trap, the nut is removed, the plug is removed, the adapter nut and washer are then used in common trade fashion to connect the trap to the trap bushing in fluid-tight fashion by way of a slip joint. The plug can be reused.