Abstract: A plastic cap is adapted for use with a bottle neck having two spaced grooves formed in the outer surface thereof the upper one of said grooves being spaced from the top edge of the neck so as to define smooth sealing surfaces above, between and below said grooves. The cap has a pair of inwardly directed circumferential beads formed on an inner surface of a thin-walled skirt that depends from a disc-shaped top of the cap. The beads are positioned so that they fit in the grooves when the cap is in place on the neck and form an interference fit with the edges of the grooves to prevent inadvertent removal of the cap. The beads are of such a size that they do not fill the grooves thereby allowing for a large contact area between the inner skirt surface and the sealing surfaces of the neck.