The present study was carried out to investigate the modulating effects of thyme and its major components against the oxidative DNA damage induced by H2O2. The human lymphocytes with thymol, carvacrol, and gamma-terpinene incubated with or without 0.1 mM H2O2 for 30 min at 37 degreesC and the DNA damage were evaluated by singe cell gel electropheresis (comet assay). Concentrations above 0.1 mM thymol and gamma-terpinene and 0.05 mM carvacrol significantly induced DNA damage in human lymphocytes, but at the smaller concentrations no additional DNA strand breakage has been observed. At the all concentrations studied, gamma-terpinene did not show any protective effect against H2O2 induced oxidative DNA damage, but the phenolic compounds thymol and carvacrol at concentrations below 0.2 and 0.1 mM, respectively, significantly reduced the oxidative DNA damage (p < 0.001). The n-hexane and ethyl acetate fractions prepared from the methanolic extracts of Thymus spicata also were found to inhibit DNA damage.