The Bacillus sp. no. AH-101 alkaline protease showed higher hydrolysing activity against insoluble fibrous natural proteins such as elastin and keratin in comparison with subtilisins and Proteinase K. The optimum pH of the enzyme toward elastin and keratin was pH 10.5 and pH 11.0-12.0 respectively. The specific activity toward elastin and keratin was 10,600 units/mg protein and 3970 units/mg protein, respectively. The enzymatic activity was not inhibited by p-chloromercuribenzoic acid and iodoacetic acid. Carbobenzoxy-glycyl-glycyl-L-phenylalanyl chloromethyl ketone completely inhibited the caseinolytic activity, but 36% elastolytic activity remained. No inhibitory effect on caseinolytic and elastolytic activity was shown by tosyl-L-phenylalanyl-chloromethyl ketone, tosyl-L-lysine chloromethyl ketone, carbobenzoxy-L-phenylalanyl chloromethyl ketone, and elastatinal. The amino acid composition and amino terminal sequence of the enzyme were determined. The no. AH-101 alkaline protease was compared with subtilisin BPN', subtilisin Carlsberg, no. 221, and Ya-B alkaline proteases. Extensive sequence homology existed among these enzymes.