Exocellular protease production was examined in two separate strains of Pseudomonas aeruginosa, one a clinical isolate and the other a laboratory strain. Both strains produced two separate proteases (proteases 1 and 2) which were indistinguishable from one strain to the other. The two proteases were purified by a two-step procedure of gel filtration chromatography followed by ion-exchange chromatography. Proteases 1 and 2 were shown to be distinct serologically and unrelated by physiochemical parameters examined. Protease 1 was the major exocellular protein produced and contributed about 95% of the total protease activity of the culture. It was etimated to have a molecular weight of 34850 and was also shown to contain 10% glucosamine by weight. Protease 2, in contrast, had an estimated molecular weight of 52750 and contained no detectable carbohydrate. Proteases 1 and 2 were both stimulated by Ca2+, and Mg2+ and inhibited by Co2+Zn2+, and 1,10-o-phenanthroline. Protease 1 was also inhibited by EDTA. In addition to protease activity, both proteases 1 and 2 demonstrated elastase activity as well as a limited collagenase activity. Specificity of the two protease against synthetic peptides was, however, quite different. Protease 1, but not protease 2, showed a preference for peptide bonds in which the amino group was contributed by an amino acid with a hydrophovic R group.