Prostromelysin, a member of the family of matrix metalloproteinases, is secreted as a zymogen which is activated after cleavage of the His81-Phe82 bond. The 82 amino acid propeptide that is removed during activation contains 12 amino acids, MRKPRC75GVPDVG, that are highly conserved in all MMPs. We evaluated a series of peptides that span this region for their ability to inhibit stromelysin. The hexapeptide, Ac-RCGVPD, and the pentapeptide, Ac-RCGVP had IC50 values of approx. 10 microM. The tetrapeptide, Ac-RCGV, was somewhat less potent with an IC50 of 60 microM. Smaller peptides, e.g. Ac-RCG, were significantly less potent as inhibitors. Substitutions of Cys75 with Ser resulted in a complete loss of inhibitory activity. The peptides in this series also inhibited human fibroblast collagenase.