In this investigation, clastogenic effects of Thymus kotschyanus var. glabrescens Boiss. extract (TE) and anticlastogenic effects of this extract against Mitomycin C (MMC) induced chromosome damage have been evaluated in human peripheral blood lymphocytes in vitro. Two series of experiments were conducted. In the first, only 10 -5 , 10 -4 , 10 -3 and 10 -2 μl ml -1 concentrations of TE were used for 48 h to detect potential clastogenicity. In the second, MMC (0.38 μg ml -1 ) plus 10 -5 , 10 -4 , 10 -3 and 10 -2 μl ml -1 concentrations of TE were used for 48 h to determine anticlastogenic effects. TE did not increase sister chromatid exchanges (SCEs) (except 10 -2 μl ml concentration) and chromosome aberrations (CAs) significantly compared with negative and solvent controls. However, it decreased the frequency of MMC induced chromosome aberrations. Decreasing was significant at 10 -4 , 10 -3 and 10 -2 μl ml -1 concentrations. On the other hand, TE significantly increased MMC-induced SCEs for all treatment groups compared with positive control. © 2006 Springer Science+Business Media, Inc.