A cap assembly comprising a body, a sealing membrane and a retaining ring. An opening is provided through the body and includes a recessed circumferential channel and a membrane engaging flange positioned proximate the recessed circumferential channel. The sealing membrane covers the opening and includes a body engaging flange and a pierceable surface. The retaining ring is positioned with the sealing membrane engagement surface in overlying engagement with the sealing membrane. Extension of the tab into the recessed circumferential channel of the body releasably maintains the overlying engagement and biases the retaining ring against the membrane. The inner wall structure of the retaining ring extends radially inward beyond the membrane engaging flange of the sealing membrane, to, in turn, protect the membrane.