A closure system for molded plastic containers having a threaded container neck, the closure system comprising a screw cap having internal threading constructed for threaded engagement with the threaded container neck, a gasket, means for retaining the gasket on the screw cap and an abutment surface integrally formed in and extending substantially radially from the container neck for sealably contacting the gasket, wherein the screw cap and the abutment surface are constructed so that downward axial rotation of the screw cap is effective to seal the gasket against the abutment surface.