DNA damage generated by oxygen-derived free radicals is related to mutagenesis, carcinogenesis and aging. In the last several years, hundreds of publications have confirmed that melatonin is a potent endogenous free radical scavenger. In the present in vitro study, we have examined the efficacy of three polyphenolic antioxidants, i.e. xanthurenic acid, resveratrol (3,4',5-trihydroxy-trans-stilbene) and (-)-epigallocatechin-3-gallate (EGCG) and two classical non-polyphenolic antioxidants, i.e. vitamin C (ascorbic acid) and alpha-lipoic acid (LA, 1,2-dithiolane-3-pentanoic acid) in inhibiting *OH-induced oxidative DNA damage. We compared the efficacy of these five antioxidants with the effectiveness of melatonin (N-acetyl-5-methoxytryptamine) and we also investigated the possible synergistic effects of melatonin with the other five molecules. Using high performance liquid chromatography (HPLC), the formation of 8-hydroxy-2-deoxyguanosine (8-OH-dG) in purified calf thymus DNA treated with the Fenton reagents, chromium(III) (as CrCl3) plus hydrogen peroxide (H2O2) (Cr(III)/H2O2), was measured in the presence or absence of the antioxidants alone or in combination with melatonin. 8-OH-dG is considered a biomarker of oxidative DNA damage. Among the antioxidants tested, melatonin was the most effective of these with an IC50 = 3.6 +/- 0.1 micro m. For the other antioxidants the IC50 values were as follows: xanthurenic acid (IC50 = 7.9 +/- 0.3), resveratrol (IC50 = 10.9 +/- 0.3), EGCG (IC50 = 5.7 +/- 0.3), vitamin C (IC50 = 16.9 +/- 0.5) and LA (IC50 = 38.8 +/- 0.7). These values differ from that of melatonin with a P < 0.01. Melatonin (1 micro M) reversed the pro-oxidant effect of resveratrol (0.5 micro M) and vitamin C (0.5 micro M), had an antagonistic effect when used in combination with EGCG (1 micro M) and it exhibited synergism in combination with vitamin C (0.5 micro M) and with LA (5 micro M).