The present invention provides a medical coupling site which is adapted to be attached directly to a standard male luer lock fitting. The coupling site includes a valve element contained within a tubular retainer. The retainer is provided with outwardly extending lugs for engaging the threaded locking collar of a male luer lock whereby the male luer lock will be held in positive mechanical locking engagement with the coupling site. The valve element is provided with rib members extending inwardly from an inner wall of the valve element toward a slit formed in a diaphragm such that the rib members act to bias the slit to a closed position. In addition, the valve element is formed with biasing means for engaging an inner wall of the retainer whereby the slit is further biased to a closed position.