To prove the relative potency of melatonin as a radical scavenger in UV-irradiated leukocytes, it was compared to other antioxidative substances such as trolox and vitamin C. Human leukocytes were isolated from EDTA whole blood and incubated with melatonin, trolox and vitamin C. The experiments were performed in a wide concentration range from 0.1 nM to 1 mM and in a small concentration range from 0.5 to 2 mM (mel), 5 mM (trolox) and 10 mM (vit. C). Irradiation was performed with UV-light (280-360 nm) at a dose of 750 mJ/cm2. Radical formation was measured by the chemiluminescence technique. The maximum effect of radical suppression was seen at a concentration of 10 nM (p = 0.003) and 1 mM melatonin (p < 0.001) and vitamin C (p = 0.002; p < 0.001), respectively. ROS suppression by trolox was only significant at 1 mM (p < 0.001). In the small concentration range, a linear dose-response relationship was found and melatonin showed the strongest radical suppression (IC50 = 0.21 mM) followed by vitamin C (IC50 = 0.26 mM) and trolox (IC50 = 1.03 mM).