A safety type cap for medicinal and similar containers capable of effecting a tight seal upon the container opening by means of a sealing flange extending laterally from the inner surface of the cap top, and which is deflected centripetally upon contact with the inner surface of the mouth of the container to result in resiliently bowing the top wall of the cap to result in increased outward pressure applied to the flange against the inner mouth surface. The radial thickness of the flange is substantially greater than the thickness of the cap top, so that the flange maintains a substantially undistorted shape during compression.