The penetration of vegetative cells of Enterobacter aerogenes, Bacillus subtilis, Micrococcus varians, and Leuconostoc mesenteraides through laser-drilled holes of diameter 10 to 20 μm in the lid of commercially manufactured semirigid aseptic cups filled with milk chocolate pudding and retort trays filled with spaghetti in meat sauce (only E. aerogenes) was studied. The ability of bacteria to pass via leakages was determined using an immersion biotest method. The threshold leakage diameter determined for both products tested was about 10 μm. The contamination rate of leaking packages was much higher for aseptic cups than retort trays. The results indicated that the factors increasing the rate of bacterial penetration into test packages are large hole size, small hole length, small bacteria diameter and leakage channels filled with liquid instead of dried foodstuff and/or bacterial suspension. The results are useful as a guideline for the requirements of on-line, nondestructive, package-integrity testers for testing packages.