Abstract: A closure structure or cap for a two compartment vial includes a resilient stopper having a cylindrical body with a lower shoulder portion adapted for sealing fit in the mouth of the vial and a second circumferential shoulder at is upper end. A substantially rigid hollow cylindrical sleeve is configured to surround the mouth of the vial, and has, at its upper end, integrally constructed downwardly projecting longitudinally split fingers dimensioned to embrace the stopper, with the bottoms of the split fingers engaging and resting upon the lower shoulder portion of the rubber stopper. An internal groove in the split fingers engages the lower edge of a second shoulder of the stopper thereby gripping the stopper. A locking member is disposed between the longitudinally split fingers and the inside walls of the hollow cylindrical sleeve or cap, forming means to prevent spreading of the fingers and thereby preventing the loosening of the grip of the fingers on the stopper.