Mutation within the Mitf gene causes, in microphthalmic Mitf(mi)/Mitf(mi) (mi/mi) mice, multiple defects, including white coat color and functional defects in macrophages and osteoclasts. Our previous mating experiments have demonstrated that the mi mutation reduces the numbers of newborns and induces uterine inversion at delivery. The present study was designed to determine the causes of these pregnancy defects. The histology and number of F4/80-positive macrophages were not different between the ovaries of 23-day-old mi/mi and +/+ mice given eCG 48 h earlier. When ovulation was induced in these mice by hCG, the number of ovulated ova was significantly smaller in mi/mi mice than in wild-type (+/+) mice (p < 0.05). When bone marrow cells from +/+ mice were transplanted i.p. into 42 mi/mi female newborns, successful transplantation was observed in 16 of them at 20 days after birth. In one of these, the upper incisors had erupted. The mean number of tubal ova in mi/mi mice significantly increased after transplantation (p < 0.05) and was almost equal to that of +/+ mice. No uterine inversion occurred at 6 deliveries in 5 mi/mi females after bone marrow transplantation, while it occurred at 4 of 5 deliveries in mi/mi females during the same observation period (p < 0.05). These results indicate that bone marrow-derived cells, defective in mi/mi mice, are necessary for normal ovulation and delivery; the findings are consistent with the notion that macrophages play major roles in ovulation.