The protective effects of carnosine as a natural dipeptide were investigated in mouse bone marrow cells against genotoxicity induced by cyclophosphamide. Mice were injected with solutions of carnosine at three different doses (10, 50 and 100 mg kg(-1) bw) for five consecutive days. On the fifth day of treatment, mice were injected cyclophosphamide and killed after 24 h. The frequency of micronuclei in polychromatic erythrocytes and the ratio of polychromatic erythrocyte/polychromatic erythrocyte + normochromatic erythrocyte [PCE/(PCE + NCE)] were evaluated by May-Grunwald/Giemsa staining. Histopathology of bone marrow was examined in mice treated with cyclophosphamide and carnosine. Carnosine significantly reduced micronucleated polychromatic erythrocytes (MnPCEs) induced by cyclophosphamide at all three doses. Carnosine at dose of 100 mg kg(-1) bw reduced MnPCEs 3.76-fold and completely normalized the PCE/(PCE + NCE) ratio. Administration of carnosine inhibited bone marrow toxicity induced by cyclophosphamide. It appeared that carnosine with protective activity reduced the oxidative stress and genotoxicity induced by cyclophosphamide in bone marrow cells of mice.