The effects of microorganism size and motility on the leak size critical to the sterility of a package, along with the imposed pressure required to initiate liquid flow for the critical leak size, were measured. Pseudomonas fragi Lacy-1052, Bacillus atrophaeus ATCC 49337, and Enterobacter aerogenes ATCC 29007 were employed to assess package sterility. One hundred twenty-six 7-mm-long microtubes with interior diameters of 5, 10, and 20 microm were used to simulate package defects. Forty-two solid microtubes were used as controls. No significant differences were found between sizes or motility statuses of test organisms with respect to loss of sterility as a result of microbial ingress into test cells with microtube interior diameters of 5, 10, and 20 microm (P > 0.05). Interactions between the initiation of liquid flow as a result of applied threshold pressures and sterility loss for test cells were significant (P < 0.05).