An investigation of the antioxidative UV protective effect of melatonin was performed in an in vitro irradiation model with leukocytes. Leukocytes were isolated from EDTA-treated whole blood and taken up in phosphate-buffered saline (PBS). Five of 10 aliquots were incubated with 2 mmol/L melatonin and 5 with PBS as a control. The samples were irradiated by UV light (280-360 nm, max: 310 nm) at doses between 75 and 300 mJ/cm(2) or left unirradiated. Radical formation was measured using the chemiluminescence technique. Staining with trypan blue was performed to assess cell viability. Melatonin significantly suppressed radical formation in cell solutions irradiated from 75 to 300 mJ/cm(2) (P </= 0.001). Controls showed an increase of reactive oxygen species (ROS) formation as a sign of oxidative stress when irradiated with increasing UV doses and a maximum ROS formation under 300 mJ/cm(2) UV light. The cytotoxicity of UV light was reduced by melatonin up to a UV dose of 1.5 J/cm(2). Leukocytes were suitable cells for the evaluation of the efficacy of melatonin as a radical scavenger under UV light. The results confirm that the clinically observed UV protective effects of melatonin may be at least partially based on its radical scavenging properties.