Collagenases (EC 3.4.24.3) from human skin, rat skin and rat uterus were inhibited by the chelating agents EDTA, 1,10-phenanthroline and tetraethylene pentamine in the presence of excess Ca2+, suggesting that a second metal ion participates in the activity of the enzyme. Collagenase inhibition by 1,10-phenanthroline could be both prevented and reversed by a number of transition metal ions, specifically Zn2+, Co2+, Fe2+ and Cu2+. However, Zn2+ is effective in five-fold lower molar concentrations (1-10(-4) M) than the other ions. Furthermore, Zn2+ was the only ion tested able to prevent and reverse the inhibition of collagenase by EDTA in the presence of excess Ca2+. Atomic absorption analysis of purified collagenase for Zn2+ showed that Zn2+ was present in the enzyme preparations, and that the metal co-purifies with collagenase during column chromatography.