The present invention relates to a cap which can form an essentially leak-proof seal with a vessel capable of receiving fluid specimens for clinical analysis and diagnosis. To minimize potentially contaminating contact between the fluid specimen and humans or the environment, the present invention features a cap which is penetrable by a plastic pipette tip or other fluid transfer device, and may include a plurality of striations which were discovered to further improve penetrability of the cap. In this way, substances can be dispensed into or withdrawn from the vessel without having to physically separate the cap from the vessel. Also featured are fluid transfer devices and caps having surface ribs and/or grooves which aid in creating passageways for venting displaced air from a penetrated collection device.