Abstract Human melanoma cells were grown to exponential and stationary phases showing melanin contents of 4.2 ± 0.3 and 11.3 ± 0.6 μg/10 6 cells, respectively. The cells were separated in four subpopulations by a Percoll gradient; the subpopulation of density 1.07 (g/ml) was the most enriched in pigmented cells and produced 28 and 58% of the cells in exponential and stationary phases, respectively. Melanoma cells had similar superoxide dismutase and glutathione peroxidase activities in exponential and stationary phases. Moreover melanoma cells exhibited a higher catalase activity in the stationary phase: whole homogenate and cytosol activities were 7.0 ± 0.3 and 10.8 ± 0.6 U/mg protein, whereas in exponential phase the activities were 4.9 ± 0.1 and 7.6 ± 0.3 U/mg protein for whole homogenate and cytosol, respectively. The intracellular H 2O 2 steady-state concentration was 3.3 ± 0.2 and 2.1 ± 0.2 μ M H 2O 2 for exponential and stationary phases, respectively. The spontaneous chemiluminescence of the two culture phases was 169 ± 27 cps/10 6 cells (exponential) and 78 ± 24 cps/10 6 cells (stationary). The cytotoxicity of H 2O 2 generated extracellularly by glucose oxidase was determined after 60 min of exposure. IC 50 values for exponential and stationary cell cultures were 0.9 and 2.4 mU/ml of glucose oxidase, respectively. The increased catalase activities in the stationary phase as compared with the exponential phase are consistent with the decreased intracellular H 2O 2, with the decreased spontaneous chemiluminescence, and with the increased resistance to exogenous H 2O 2.