Previously reported experiments (Winand, R.J., and Kohn, L.D. (1970) J. Biol. Chem. 245, 967-975; Kohn, L.D., and Winand, R.J. (1971) J. Biol. Chem 246, 6570-6575) have demonstrated that partial pepsin digestion of bovine thyrotropin preparation yields a fragment of the thyrotropin molecule which is