Protective effect of quercetin, a natural antioxidant compound, on hypericin-induced cytotoxicity was studied in human promyelocytic leukemia cells (HL-60). Hypericin (10(-5) mol x l(-1)) alone significantly decreased cell survival to 21% that found in the controls, whereas in combination with quercetin (10(-5) mol x l(-1)) this decrease was diminished to 46%. Lower concentrations of quercetin had no protective effect. These findings indicate that oxygen radicals can play an important role in hypericin-induced phototoxic effects.