Vitamin C has been suggested1,2 and disputed3 as an anti-cancer agent. For cells in culture, no preferential effect against any type of cancer has yet been demonstrated. Our aim here is to show that vitamin C is selectively toxic to at least one type of malignant cell—a melanoma—at concentrations that might be attained in humans. Copper ions react with ascorbate and generate free radicals in solution4. Ascorbate when combined with copper rapidly reduces the viscosity of DN A solutions and has exhibited some carcinostatic effects on transplanted sarcoma 180 tumours in mice5. We reasoned that the elevated copper concentration in melanoma6,7 could result in a more selective toxicity for ascorbate.