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Aeromonas liquefaciens and Aeromonas salmonicida were both found to produce considerable amounts of extracellular proteinases, when grown on semi-solid skim milk agar at 30° C. The proteinase A, produced by Ae. liquefaciens only, and the proteinase B, produced also by Ae. salmonicida were purified approximately 100 times by precipitation twice with (NH4)2SO4, batchwise treatment with DEAE-cellulose and, finally, gel filtration on Sepha-dex G-100. The pH-optimum was estimated to be 7.9 for proteinase A and 9.0 for proteinase B. Differences between the proteinases were also found with regard to thermoresistance, and to their behaviour in the presence of some naturally occurring proteinase inhibitors. The addition of iron and cobalt ions increased the activity of both proteinases. Molecular weights were estimated to be 22 100 for the proteinase A and 43 600 for proteinase B, on the basis of gel filtration on Sephadex G-100.