Organism. P. aeruginosa PA103, isolated by Liu (23), was selected for this study. This strain is naturally elastase deficient (28). We found that all protease activity in culture supernatants of strain PA103 was neutralized by specific alkaline protease antisera, demonstrating that the only extracellular protease produced by this strain is alkaline protease. This strain and derived mutants were maintained on nutrient agar plates and stored in 10% skim milk at -70°C. Where indicated, spontaneous mutants resistant to streptomycin and rifampin were selected by growth on 250 [.g of streptomycin (streptomycin sulfate; Sigma Chemical Co., St. Louis, Mo.) per ml, followed by growth on 50 jig of rifampin (Sigma) per ml.