The genotoxic effects of thymol were investigated in human peripheral lymphocytes treated with 25, 50, 75, and 100 mu g/ml concentrations of thymol for 24 and 48 h treatment periods by using sister chromatid exchange (SCE), chromosome aberration (CA), and micronucleus (MN) tests. Nuclear division index replication index (RI), and mitotic index (MI) were also calculated in order to determine the (NDI), cytotoxicity of thymol. Thymol significantly increased the SCE, especially at the lower concentrations. Thymol also increased the SCE at the highest concentrations without statistical significance. Thymol induced both the structural CA and frequency of MN at all concentrations. Thymol dose-dependently decreased the NDI for two treatment periods. Thymol decreased the RI for the 24h treatment time without any statistical significance. However, thymol decreased the RI for the 48 h treatment time in a dose-dependent manner. Thymol also decreased the MI at the higher concentration without dose-dependent effect. Published by Elsevier Inc.