Abstract Reactive oxygen species (ROS) are potentially cytotoxic and several mechanisms have evolved to protect against their damaging effects. In melanocytes, tyrosinase may have such a role by utilising the superoxide anion (O2−)in the production of melanin. In the present study, we have examined the cytotoxic effects of O2− and hydrogen peroxide (H2O2) in human melanocytes both before and following the activation of tyrosinase. Xanthine oxidase (XO, 5–150 mU · ml−1) and glucose oxidase (GO, 0.1-20 mU · ml−1) were used to generate the O2− and H2O2 respectively, and the cytotoxic effects assessed by measuring cell survival using the 3-[4,5-dimethylthiazol-2-yl]-2.5 diphenyltetrazolium (MTT) assay. 3 h later, dose related decreases in melanocyte survival were seen. Similar experiments with keratinocytes and libroblasts showed that these cells were more resistant to the cytotoxic effects of O2− than were the melanocytes. The effect of increasing tyrosinase activity was examined by growing the melanocytes in the presence of an analogue of melano-cyte-stimulating hormone (MSH) Nle4DPhe7α-MSH (10−8 M), for 48 h. This increased tyrosinase activity, melanin content, the ability to trap O2− and the resistance of the melanocytes to the cytoloxic effects of this ROS. but failed to alter their susceptibility to the damaging effects of H2O. Nle4DPhe7α-MSH had no effect on the resistance of keratinocytes and libroblasts to either O2− or H202. After 3 h, XO. as opposed to GO. also increased the melanin content of human melanocytes; this effect was not accompanied by an increase in tyrosinase activity. The present results suggest that tyrosinase may utilise O2− to produce melanin and that this process may protect melanocytes from the potentially damaging effects of this ROS.