PURPOSE The corneal destruction resulting from P. aeruginosa keratitis is thought to be due, in part, to the actions of host and bacterial proteases on corneal tissue. The intent of the current study was to compare host and bacterial protease expression in the ocular tissues of mice that have been shown to be susceptible to corneal infection with mice that have been protected from such infection as a result of prior immunization. METHODS Supernatants of infected corneal homogenates from naive mice or from mice immunized as a result of prior corneal infection or that had been immunized against bacterial protease were analyzed by gelatin zymography for the presence of host gelatinases and bacterial proteases at various times after infection. In addition, studies were carried out to examine the effect of immunization with bacterial protease on numbers of ocular bacteria. RESULTS Mice that were protected against the tissue destruction during infection were found to have reduced proteolytic activity in corneal tissues when compared with susceptible mice. More specifically, the resistant mice displayed lower levels of bacterial alkaline protease, latent MMP-2, and both the active and latent forms of MMP-9. In addition, immunization against either bacterial alkaline protease or elastase failed to reduce bacterial numbers in ocular tissues from 6 to 72 hours after infection. CONCLUSIONS These results suggest that increased proteolytic activity in ocular tissues during P. aeruginosa infection may contribute to the irreversible corneal damage observed during the infection.