The present invention provides a medical coupling site (210) which is adapted to be directly attached to a standard male luer fitting. The coupling site includes a valve element (216) contained within a tubular housing (214). The housing (214) is provided with a threaded portion 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 (210). The valve element (216) is provided with rib members (244, 246) extending inwardly from an inner wall of the valve elements (216) toward a slit (236) formed in a diaphragm (234) such that the rib members (244, 246) act to bias the slit (236) to a closed position. The housing (214) includes a flange extending radially inwardly over the diaphragm (234) and defining a diameter for frictionally engaging a male luer in locking engagement. In addition, an exposed surface (249, 251) of the diaphragm (234) is formed with surface texturing to define a roughened surface (249, 251). The roughened surface (249, 251) reduces the surface friction between the diaphragm (234) and a male luer as the luer is inserted through the diaphragm (234).