Tannic acid has numerous food and pharmacological applications. It is an additive in medicinal products, and is used as a flavouring agent and as an anti-oxidant in various foods and beverages. We have previously shown that tannic acid in the presence of Cu(II) causes DNA degradation through generation of reactive oxygen species. On the other hand, it exhibits antimutagenic and anticarcinogenic activities, and induces apoptosis in animal cells. It is known that most plant-derived polyphenolic anti-oxidants also act as pro-oxidants under certain conditions. In this paper, we compare the anti-oxidant and pro-oxidant properties of tannic acid and its structural component gallic acid. It is shown that tannic acid is the most efficient generator of the hydroxyl radical in the presence of Cu(II), as compared with gallic acid and its analogues syringic acid and pyrogallol. The anti-oxidant activity of tannic acid was studied by its effect on hydroxyl radical and singlet oxygen mediated cleavage of plasmid DNA. Again, tannic acid provided the maximum protection against cleavage, while gallic acid and its structural analogues were found to be non-inhibitory or partially inhibitory. The results suggest that the structural features of tannic acid that are important for its anti-oxidant action are also those that contribute to the generation of hydroxyl radicals in the presence of Cu(II). Restriction analysis of treated phage DNA and thermal melting profiles of calf thymus DNA indicated that tannic acid strongly binds to DNA. Indirect evidence indicates that modification of DNA bases may also occur.