Abstract: To prevent tampering with the contents of containers for pharmaceuticals and other products, a plastic cap which is an improvement upon U.S. Pat. Nos. 3,338,446 and 4,166,552. The neck of the container is sealed with a commercially available foil seal; the cap is dimensioned so that the foil seal is inserted inside the cap prior to its being applied to the container. The dimensions of the cap accommodate the foil seal yet the snap-on and the tamper-proof features of the cap are maintained. At intervals, scorelines are formed extending upward from the bottom of the cap skirt; these tear if one attempts to pry the cap off the container and are visible evidence of the tampering.