Bivalve molluscs, particularly mussels, are sensitive biomarkers of aquatic ecosystem pollution. The tannins, water-soluble plant polyphenols, may play an important role in this environment and, mainly as a consequence of interaction with pollutants, their toxicity may change. We studied three naturally occurring compounds, tannic acid, ellagic acid and gallic acid, for their ability to modulate DNA damage produced by these tannins alone and in the presence of the oxidative stress inducer H₂O₂, in cells of the digestive gland of mussels (Unio tumidus). After the treatment of the cells with polyphenols at different concentrations (1, 5, 15, 30, 60, 80, 100, 120, 180, 240 μM) and with hydrogen peroxide in the range of 0.04 and 0.1 mM, single-strand breaks (ssb) in DNA were investigated, using the comet assay. The ability of phenolic acids to decrease DNA damage through their antioxidant properties was also assessed. The results show that the phenols, which are known as antioxidative agents, could also act as pro-oxidants. They induced ssb in DNA of the digestive gland at concentrations higher that 10 μM, but lower doses (1 and 5 μM) did not contribute to the DNA damage. This study was also designed to evaluate the protective effect of these tannins against H₂O₂-mediated DNA damage in the cells. In this treatment, the two concentrations (1 and 5 μM) significantly decreased the amount of lesions induced by H₂O₂ (0.04 and 0.1 mM). In conclusion, our results demonstrate that antioxidative properties of tannins may change to pro-oxidative activities at the higher concentrations. This suggests that the biologic actions of these compounds may be rather complicated.