The effect of WR-2721 against cyclophosphamide-induced clastogenicity was studied using the in vivo micronucleus assay. The frequency of micronuclei in polychromatic erythrocytes in the peripheral blood of mice treated with WR-2721 and cyclophosphamide (CP), each of the compounds at a dose of 200 mg/kg body weight, was evaluated during the 15-day period. The suppressing effect of WR-2721, given 30 min prior to cyclophosphamide administration, on micronuclei induced by the alkylating agent was demonstrated. The number of micronuclei was increased on day 1 after CP application and declined thereafter with the frequency of micronucleated polychromatic erythrocytes remaining lower in WR-2721 pre-treated mice. The modulatory effect of WR-2721 on the clastogenic activity of cyclophosphamide in the erythropoietic system by the mouse micronucleus test was shown.