As part of the 9th collaborative study by CS-GMT/JEMS-MMS, the possible human carcinogens (IARC group 2B) hydrazine (HZ) and 1,2-dimethylhydrazine (SDMH) were evaluated in the micronucleus test using rats. HZ was intraperitoneally injected twice or four times at 24-h intervals into male SD rats; SDMH was administered daily by oral gavage, twice, to male F-344 rats. Femur bone marrow cells and peripheral blood were collected, and 2000 immature erythrocytes were analyzed and the frequency of micronuclei recorded.HZ induced micronucleated immature erythrocytes in bone marrow and peripheral blood statistically significantly, although the magnitude was low. In mice, four HZ treatments, but not the single treatment, were found to enhance micronucleus induction in immature erythrocytes but this was not the case in rats. SDMH induced micronucleated immature erythrocytes in both bone marrow and peripheral blood significantly. Because these results contradict the data published for Alpk rats, SDMH can be regarded as a representative chemical that shows strain differences in the induction of micronuclei. The reason for these discordant results is not clear, and further study is needed to give an explanation.