Abstract: The authors reported a method of detecting micronuclei in peripheral normochromatic erxthrocytes(NCE)in mice. Male mice was divided into 3 groups: cyclophosphamide(CP) group, mitomycin(MMC)group and bone marrow control group. The treatment continued for 5 weeks. The rate of micronucleated NCE in peripheral blood was increased significantly after only 5 injections for each of the two chemicals. The micronucleus rate increased consistently with time. Following rates were observed at 5-th week: CP(22 mg/kg), 19.3/1000NC (N=6), MMC(0.5mg/kg). 29.7/1000NCE(N=6) and control 1.83/1000NCE (N=6), while those for polychromatic erythrocytes (PCE) in bone-marrow of mice treated by CP (22mg/kg) and MMC(1.0mg/kg)are 2-3.5 and 2.5-4.5/1000PCE, resp. The rate of micronucleated NCE at 5-th week after begining of treatment was 3 to 6 fold higher than that of administration for one week. Since micronucleated erythrocytes Survive in the peripheral circulation for about 1 month micronucleus rate resulting from repeated treatment is additive it is more sensitive than the traditional micronuclei test method.