Metastasis is a complicated multi-step process involving interactions between tumour cells, the extracellular matrix and the vessel walls. Experimental observations suggest that leucocyte migration and function could be a suitable model in order to understand tumour cell dissemination. In the present report we show and quantify the production of free radicals by human malignant melanoma cells (St-ml12) by means of a spectrophotometrical method, using an enzyme immunoassay reader. Endothelial cells and activated polymorphonuclear leucocytes were used as controls. Melanoma cells without stimulants produced large amounts of superoxide anion at an increasing rate in relation to time, which could be inhibited by superoxide dismutase. Production of hydrogen peroxide was minimal. The endothelial cells produced a negligible amount, in contrast to the activated polymorphonuclear leucocytes, which released large quantities of both free radicals. A rapid assay to analyse the production of free radicals by tumour cells is presented here. Using this, we demonstrated that melanoma cells produce superoxide anions, supporting previous observations which implicate superoxide anion in the mechanism of metastasis.