Reactive oxygen species (ROS) are presumed to be involved in inflammatory UV reactions of the skin. This in vitro study was performed to investigate the suppressive effect of melatonin in interleukin-3 (IL-3) stimulated leukocytes. Neutrophilic granulocytes were isolated from EDTA-treated whole blood and placed in a phosphate-buffered saline (PBS) containing IL-3. Cell suspensions were either treated with PBS (control) or with increasing doses of melatonin (0.1, 0.5, 1, 2, 3, 5, 7.5, 10 mmol). One PBS solution was left unirradiated and the other nine solutions (PBS and melatonin) were irradiated with 750 mJ/cm2 UVB light (280-360 nm, max: 310 nm). Radical formation was measured by the chemiluminescence technique. UV-irradiated leukocytes showed a 5-fold higher radical formation than unirradiated leukocytes. Melatonin, in increasing doses in powers of ten, led to a maximum suppression of free radicals at 10 nmol (P= 0.01) and 1 mmol melatonin (P = 0.001), showing a biphasic, non-linear, dose-response relationship. Melatonin, given in amounts of 0.1-10 mmol, led to a direct dose-dependent suppression of ROS. Radical formation was suppressed significantly in a range from 0.5 to 10 mmol (P = 0.001). Melatonin is known to function as a radical scavenger and antioxidant; some of these melatonin effects may be receptor independent, while others may be receptor dependent.