A sealing cap (2, 2a) is mounted to the open mouth of a pharmaceutical-containing container (20) which is to undergo lyophilization. The container has an outwardly extending mouth ring (26) with a shoulder (28) spaced apart from and facing away from the mouth (22). The sealing cap includes a body (4, 4a) having a through hole (6) covered by a piercible septum (16). The body includes radially deflectable fingers (30, 30a) which engage the mouth ring when the cap is at an open position, at which fluid flow into and out of the interior (38) of the container is substantially unhindered. The fingers also include surfaces (42, 42a) which engage the shoulder of the mouth ring when the cap seals the container mouth. The cap is locked into place using a lock ring (44, 44a) which engages the distal ends (47, 47a) of the fingers to keep the fingers engaged beneath the mouth ring. The sealing cap moves from the open to the sealed position using straight line, axial movement with simple fixtures.