Isolation of mutants specifically deficient in the production of alkaline protease would facilitate definitive studies addressing the role of alkaline protease in P. aeruginosa infections. This approach has been complicated by the fact that most strains of P. aeruginosa produce at least two distinct extracellular proteases (25) and that alkaline protease does not possess a unique or stringent substrate specificity (26). We have been unable to specifically inhibit the activity of interfering protease (elastase) to facilitate detection of alkaline protease mutants on skim milk agar plates (unpublished data). To circumvent these problems, we chose to isolate alkaline protease mutants in strain PA103, which produces alkaline protease (4) but no detectable elastase (28). Despite the fact that strain PA103 is serum sensitive (41), it is virulent in a mouse eye model of corneal infection (17, 27). We have isolated alkaline protease-deficient mutants of strain PA103 after ethyl methanesulfonate (EMS) mutagenesis and screening of mutagenized colonies on skim milk agar plates. After in vitro characterization of these mutants, mutants specifically deficient in alkaline protease production were identified, and the virulence of these mutants was compared with that of the wild-type strain in a mouse eye model.