Membrane-bound alkaline proteases from the midgut epithelia of the silkworm, Bombyx mori, were solubilized with 1% Lubrol-WX, at pH 11.2. They were purified by gel filtration on Sepharose 6 B and Ultrogel AcA-202 columns and a preparative polyacrylamide gel electrophoresis. Two proteases, caseinolytic (6 B 3-Tc) and benzoyl-arginine-p-nitroanilide-lytic (6 B 3-Tb) were obtained. Both enzymes were homogeneous as judged by polyacrylamide electrophoresis. These enzymes showed high pH optima, 11.2, and pI values, above 11, and were extremely stable over a wide range of pH. The K_m values for 6 B 3-Tb and Tc were 0.476mM and 2.5mg/ml respectively. Hammarsten casein and mulberry leaf protein were rapidly hydrolyzed by Tc, whereas the hydrolytic activity of Tb for Azocoll was higher than that of Tc. The protease Tb was strongly inhibited by diisopropylfluorophosphate, p-chloromercuribenzoate, benzamidine, leupeptin, and soybean trypsin inhibitor; Tc was inhibited by diisopropylfluorophosphate, tosyl phenylalanine chloromethylketone and chymostatin, but not by tosyl lysine chloromethylketone, p-chloromercuribenzoate, or iodoacetamide. The molecular weights of the proteases were estimated to be 12, 800 (Tb) and 13, 300 (Tc) by Sephacryl S-300 gel filtration. The amino acid analyses showed that both proteases contain a large number of acidic amino acids but a relatively small number of basic amino acids.