Cytogenetic studies were done on 34 primary leukemias induced by N-nitroso-N-butylurea (NBU) in outbred Long-Evans rats. The results revealed leukemia cells with No. 2 trisomy and long No. 2 chromosome, which characterized the leukemias induced by polycyclic hydrocarbon carcinogens such as 7,12-dimethylbenz[a]anthracene and 7,8,12-trimethylbenz[a]-anthracene. These findings suggest a common mode of action of different carcinogenic chemicals at the chromosome level, although the lower incidence of these chromosome changes and of erythroblastic leukemias with NBU suggests subtle difference in their actions.