Pseudomonas aeruginosa strains have been assayed for endopeptidase activities in cultures of recently isolated variants, as well as in cultures from two-months-old variants derived from the same strains. Using chromogenic peptides, information was gained on both extracellular and cell-bound endopeptidases. Production of extracellular endopeptidases detected in the recent isolates tended to decrease or was abolished in the two-months-old sub-cultures. Their pattern of activities also showed some distinct differences between the strains. By means of caseinate precipitation, the production of extracellular endopeptidases was found to present important variations, both according to the strain, and according to the duration of the incubation time. The cell-bound endopeptidases were found to be relatively stable features, independent of the time passed since isolation. Their pattern of activities was also rather similar among the strains.