Stromelysin is secreted as an inactive zymogen that is activated in the extracellular space by cleavage of the His81-Phe82 bond with the release of the 81-amino acid propeptide domain. This segment contains a 12-amino acid sequence (MRKPRC75GVPDVG) that is highly conserved in all matrix metalloproteinases. Previous studies have shown that the hexapeptide, Ac-RCGVPD-NH2, and the pentapeptide, Ac-RCGVP-NH2, based on this region retain significant inhibitory activity. This new structure-activity relationship study of both peptides has shown that only Cys75 and Val77 are essential for inhibitory activity. Peptides based on this series inhibited stromelysin and collagenase with equal potency. Additional peptides spanning this region were synthesized in order to focus on these two sites. Significantly, isocysteine was substituted for Cys75 without significant loss of inhibitory activity. Tyr-(2,6-dichlorobenzyl) was substituted for Val77. The introduction of these 2 new residues into Ac-CGVP-NH2 produced a very potent inhibitor, Ac-isoCGY-(2,6 dichlorobenzyl)-P-NH2 with an IC50 of 3 microM. The following factors, acting in combination, determine the inhibitory activity of peptides in this series: distance between the sulfur atom and the peptide backbone, coordination geometry of the thiol side chain with the active-site zinc, and conformational flexibility of the side-chain.