The frequency of micronucleated polychromatic erythrocytes in the bone marrow of Swiss mice treated with WR-2721, at a dose of 200 mg/kg or 400 mg/kg body weight, 15 or 30 min prior to cyclophosphamide (CP) administration, at a dose of 200 mg/kg body weight, was determined 24 h after CP treatment. In mice injected with CP, the number of micronuclei in polychromatic erythrocytes was significantly increased in comparison with the controls, and in mice treated with WR-2721 and CP, the frequency of micronucleated polychromatic erythrocytes was distinctly decreased in comparison to those given CP alone. The protective effect of WR-2721 against cyclophosphamide-induced clastogenicity was shown. The effect was dependent on the dose of the thiol agent given, and it was more expressed when WR-2721 was applied at the higher dose, 400 mg/kg body weight. However, the protection by the aminothiol appeared not to depend on the time intervals between WR-2721 and CP administration to the mouse organism.