The chromosome aberration assay in the bone marrow cells of C57B1/6 mice showed that the new 2-mercaptobenzimidazole derivative afobazole(1-100 mg/kg) prevented manifestations of the clastogenic effects of dioxidine (100 mg/kg, i.p.) over a period of 24 h and reduced by 44-75% the cytogenetic effect of dioxidine (300 mg/kg, i.p.). The same doses of afobazole produced a statistically significant decrease in the cyclophosphamide (20 mg/kg, i.p.) damage over a period of 24 h. Afobazole showed no inherent mutagen activity and did not potentiate the effects of mutagens studied.