The reaction of pig pepsin and pepsinogen with 3H-dicyclohexylcarbodiimide (DCC-T) is studied. It is found that about three residues of DCC-H incorporate into pepsin molecule and about four residues--into pepsinogen molecule under the excess of carbodiimide. The incorporation of DCC-T into protein results in a deep enzyme and zymogen inactivation: pepsine retains 20% of the proteolytic activity with respect to hemoglobin, and pepsinogen completely loses its potential proteolytic activity. It is found that a part of DCC-T residues attaches to N-terminal part of pepsinogen, which splits off under the activation of proenzyme in acid medium. It is demonstrated that the interaction of DCC-T with carboxyl groups of dicarbonic amino acids results in the change of isoelectric point of pepsin and pepsinogen.