AbstractThe micronucleus test is used widely as an in vivo short‐term assay for potential carcinogens. In the present study, results of the micronucleus test were affected by the rate of erythropoiesis in the bone marrow erythropoietin, a growth factor for the erythroblast, which was used to induce erythropoiesis.The highest frequency of micronucleated polychromatic erythrocytes (MPCE) and a dose‐response relationship between erythropoietin doses and MPCE frequency were seen 30 hr after injection of 1,1‐dimethylhydrazine (DMH) to mice administered 24 hr previously with erythropoietin. The effect of erythropoietin was maximal when erythropoietin was given 24 hr before DMH, indicating that accelerating the multiplication of erythroblasts will increase the frequency of micronuclei induced by mutagens.Induction of MPCE in the bone marrow by four other compounds—benzo(a)pyrene,2‐naph‐thylamine, mitomycin C, and vincristine—was also increased by pretreatment with erythropoietin.