Both melatonin and its precursor N-acetylserotonin have been reported to exert antioxidant properties both in vitro and in vivo. Since little is known about their antioxidant activity in lymphocytes, we investigated their effects on spontaneous and on oxidant-induced reactive oxygen species formation in human peripheral blood lymphocytes in comparison to the antioxidant trolox, a water-soluble analogue of alpha-tocopherol. Both melatonin and N-acetylserotonin exhibited antioxidant properties against t-butylated hydroperoxide- and diamide-induced reactive oxygen species formation in peripheral blood lymphocytes. N-acetylserotonin turned out to be about three times more effective than melatonin. In resting cells, the intracellular reactive oxygen species concentration was only decreased by N-acetylserotonin and trolox, melatonin had no effect. In t-butylated hydroperoxide-mediated cell death, N-acetylserotonin was as effective as trolox in protecting peripheral blood lymphocytes from cell death and required 10-fold lower concentrations than melatonin. Furthermore, in an aqueous cell-free solution, the capacity of N-acetylserotonin to scavenge peroxyl radicals was much higher than that of melatonin. These results clearly indicate N-acetylserotonin to be a much better antioxidant than melatonin.