Abstract: A closure structure for a two-compartment vial having an open neck at one end thereof with an annular rim encircling the neck at the free end thereof. The closure structure includes a stopper which extends into the neck and has a protruding portion of reduced diameter. The closure structure also includes a cap having a gripping portion sleeved upon the neck for gripping the rim. A sleeve portion of the cap snugly and slidably embraces the protruding portion of the stopper, and is connected to the gripping portion by a fracture region. The sleeve portion is movable with the protruding portion into the neck. Interengageable lock structure is provided on the sleeve and protruding portions for positively resisting inward movement of the stopper relative to the sleeve portion.