Recently, copper has been shown to be capable of mediating the activation of several xenobiotics producing reactive oxygen and other radicals. Since copper exists in the nucleus and is closely associated with chromosomes and DNA bases, in this study we have investigated whether the activation of 1,4-hydroquinone (1,4-HQ) and a variety of other phenolic compounds by copper can induce strand breaks in double-stranded phi X-174 RF I DNA (phi X-174 relaxed form I DNA). In the presence of micromolar concentrations of Cu(II), DNA strand breaks were induced by 1,4-HQ and other phenolic compounds including 4,4'-biphenol, catechol, 1,2,4-benzenetriol, 2-methoxyestradiol, 2-hydroxyestradiol, diethylstilbestrol, butylated hydroxytoluene, butylated hydroxyanisole, tert-butylhydroquinone, ferulic acid, caffeic acid, chlorogenic acid, eugenol, 2-acetamidophenol, and acetaminophen. Structure-activity analysis shows that in the presence of Cu(II), the DNA cleaving activity for phenolic compounds with a 1,4-hydroquinone structure, such as 1,2,4-benzenetriol and tert-butylhydroquinone is greater than those with a catechol group (catechol, 2-hydroxyestradiol and caffeic acid). Those compounds having one phenol group, such as eugenol, 2-acetamidophenol, and acetaminophen, are the least reactive. In addition, the induced DNA strand breaks could be inhibited by bathocuproinedisulfonic acid, a Cu(I)-specific chelator, or catalase indicating that a Cu(II)/Cu(I) redox cycle and H2O2 generation are two major determinants involved in the observed DNA damage. Using reactive oxygen scavengers, it was observed that the DNA strand breaks induced by the 1,4-HQ/Cu(II) system could not be efficiently inhibited by hydroxyl radical scavengers, but could be protected by singlet oxygen scavengers, suggesting that either singlet oxygen or a singlet oxygen-like entity, possibly a copper-peroxide complex, but not free hydroxyl radical probably plays a role in the DNA damage. The above results would suggest that macromolecule-associated copper and reactive oxygen generation may be important factors in the mechanism of 1,4-HQ and other phenolic compound-induced DNA damage in target cells.