Abstract Neutrophils which accumulate at sites of inflammation secrete a number of injurious oxidants which are highly reactive with protein sulfhydryls. The present study examined the possibility that this reactivity with thiols may cause protein damage by mobilizing zinc from cellular metalloproteins in which the metal is bound to cysteine. The ability of the three principal neutrophil oxidants, hypochlorous acid (HOCl), superoxide (· O 2 −), and hydrogen peroxide (H 2O 2), to cleave thiolate bonds and mobilize complexed zinc was compared using two model compounds (2,3-dimercaptopropanol and metallothionein peptide fragment 56–61), as well as metallothionein. With all compounds, 50 μ m HOCl caused high rates of Zn 2+ mobilization as measured spectrophotometrically with the metallochromic indicator 4-(2-pyridylazo)resorcinol. Xanthine (500 μ m) plus xanthine oxidase (30 mU), which produced a similar concentration of · O 2 −, also effected a rapid rate of Zn 2+ mobilization which was inhibited by superoxide dismutase but not catalase, indicating that · O 2 − is also highly reactive with thiolate bonds. In contrast, H 2O 2 alone was much less reactive at comparable concentrations. These data suggest that HOCl and · O 2 − can cause damage to cellular metalloproteins through the mobilization of complexed zinc. In view of the essential role played by zinc in numerous cellular processes, Zn 2+ mobilization by neutrophil oxidants may cause significant cellular injury at sites of inflammation.