The parenteral vial/closure seal interface is tested for leakage of gas, liquids, and microorganisms. Gas leakage is determined using a bubble test, liquid leakage is detected by atomic absorption of a copper ion tracer solution, and microbial leakage is verified using a liquid-borne microbial suspension challenge test. Leakage performance by these methods is compared to gaseous leakage rates quantitatively determined for the same package systems. The establishment of leakage rate specifications based on these results is discussed.