The C-terminal alpha-amidation of peptides is one of the most important events in prohormone and neuropeptide processing. Peptide amidation is a two-step process catalyzed by peptidylglycine (hydroxylating) monooxygenase (B. A. Eipper et al., 1983, Proc. Natl. Acad. Sci. USA 80, 5144-5148) followed by dismutation of the resultant hydroxylated peptide to peptide amide and glyoxylate, stimulated by alpha-hydroxyglycine amidating dealkylase (K. Takahashi et al., 1990, Arch. Biochem. Biophys. 169, 524-530). Previous reports on peptidylglycine monooxygenase from bovine pituitary have generated substantial disagreement as to its molecular size. We have reinvestigated the purification of this enzyme and we find that peptidylglycine monooxygenase activity from fresh bovine pituitary is entirely due to a previously unrecognized catalytic function of growth hormone (somatotropin).