Summary Streptomyces griseus Protease B is a close homologue of Protease A, another serine protease isolated from Pronase. Homology based on identity of residues in the two enzymes is 61%. Extensive identical sequences are found in the vicinities of histidine-57, aspartic acid-102, serine-195, the two disulfide bridges, the NH2-terminal and COOH-terminal ends as well as in the region of the presumed substrate binding sites. However, certain regions of the Protease B sequence are markedly different and include a heptapeptide insertion between residues 88 and 89 and a tetrapeptide deletion between residues 129 and 136 when compared with Protease A. These differences are probably responsible for the remarkable stability of Protease B in concentrated solutions of urea and guanidine hydrochloride.