The effects of prostaglandin E2 (PGE2), as a trigger of erythroid progenitor cells into the cell cycle, were studied on the induction of micronuclei by various mutagens; with mitomicin C (MMC) the optimal protocol was established. PGE2 itself did not induce any micronuclei in this experiment. The highest frequency of micronuclei and dose-response relationship between PGE2 doses and micronucleus frequency were observed 30 h after injection of MMC to mice administered PGE2 24 h previously. Sensitization by PGE2 pretreatment was also found for other mutagens, such as vincristine, 5-fluorouracil, benzo[a]pyrene, 1,1-dimethylhydrazine and 2-naphthylamine. These results support the hypothesis that accelerating the erythropoiesis increases the frequency of micronuclei induced by mutagens.