Abstract: An improved tamperproof closure system for molded plastic containers has a threaded neck portion sealed at its distal end, and a screw-cap designed for attachment to the container neck. A flange extends radially from the container neck and a frangible annular groove is circumferentially disposed within the flange. A shoulder on the screw-cap is designed to press against the flange when the screw-cap is rotated downwardly on the neck. The force of the shoulder pressing against the flange ruptures the frangible annular groove, thereby severing the end portion of the container neck from the remainder of the neck, and retaining it within the screw-cap. A stop mechanism positioned in the threading of the container neck allows the screw-cap to be rotated downward to a point short of that necessary to rupture the frangible annular groove and yet far enough to securely attach the cap to the container. The stop mechanism also indicates when a container has been opened.