The present report extends a preceding investigation of the isolation, characterization, and activation of bovine pancreatic procarboxypeptidase B (Wintersberger et al., 1962). The amino acid composition of the zymogen and of crystalline carboxypeptidase B derived from it is given in detail. The amino terminal residue of carboxypeptidase B has been identified as threonine. The initial stage of activation of procarboxypeptidase B has been examined, and evidence has been obtained that the appearance of activity coincides with the hydrolysis of a single arginyl-threonine linkage in the zymogen. Preliminary data indicate that crystalline bovine carboxypeptidase B, like carboxypeptidase A and porcine carboxypeptidase B, contains approximately one gram atom of zinc per mole (molecular weight 34,000).