Two proteolytic enzymes of Pseudomonas aeruginosa--an alkaline protease and an elastase--were incubated with human myeloma proteins IgG and IgA as well as with secretory IgA at 37 degrees C. Digest mixtures were analyzed after 1, 5, 12, 24, 48 and 72 h by SDS-polyacrylamide gel electrophoresis after reduction by 2-mercaptoethanol. Under conditions which resulted in cleavage of all three immunoglobulins by the elastase in the hinge-region, the alkaline protease cleaved only IgA. It was suggested that proteases of PA interfere with the immune-system of the host by cleavage of immunoglobulins. Elastase-positive PA strains should be more virulent compared with PA strains which produce only alkaline protease or are protease-negative at all.