Peptides that display bradykinin-potentiating activity have been obtained from a number of distinct sources, such as snake venoms, fibrinogen, and casein. This paper describes the isolation and sequencing of a novel bradykinin-potentiating peptide, generated by tryptic hydrolysis of the gamma-casein chain. No homology was found to other known vasoactive or vasopotentiating peptides. The octapeptide Tyr-Pro-Val-Gln-Pro-Phe-Thr-Glu, corresponding to the gamma-casein(114-121) sequence, was isolated from the tryptic hydrolysis of gamma-casein and also synthesized by solid-phase peptide synthesis. Both natural and synthetic peptides had the same retention time in HPLC and displayed a selective potentiating activity on isolated guinea-pig ileum for bradykinin and Lys-bradykinin but were not able to potentiate the effects of Met-Lys-bradykinin, Ile-Ser-bradykinin, angiotensin II, acetylcholine, or histamine. Intravenous injections of bradykinin and of bradykinin-potentiating octapeptide produced a persistent hypotension in conscious rats, a pattern that was not obtained when the octapeptide was replaced by captopril. This bradykinin-potentiating octapeptide is a strong competitive inhibitor of endo-oligopeptidase A (EC 3.4.24.15, formerly EC 3.4.22.19), but it has low inhibitory potency towards angiotensin-converting enzyme (EC 3.4.15.1). Thus, our results suggest that other peptidases in addition to angiotensin-converting enzyme, such as endo-oligopeptidase A, may contribute to the reduction of the effective concentration of bradykinin in the circulation.