Correlative evidence suggests that maternal production of the mononuclear phagocyte growth factor colony stimulating factor-1 (CSF-1) regulates placental development. In order to study the role of CSF-1 in pregnancy the fertility of CSF-1-less osteopetrotic (op op) mutant mice was investigated. Homozygous mutant crosses (op op× op op) were consistently infertile. As expected, op op males were almost completely fertile when crossed with heterozygous females. Surprisingly, op op females when mated to heterozygote males were fertile, although at a rate that was 46% of the rate for+ op females× op op males. These data suggest that CSF-1 is required for pregnancy. However, a maternal CSF-1 source is not absolutely necessary in that pregnancies involving+ op fathers were partially rescued, suggesting that+ op fetuses and/or+ op seminal fluid provides CSF-1 or CSF-1-induced factors which compensate for the absence of maternally produced CSF-1. Despite the complete absence of CSF-1 in the uterus and placenta of op op mice placental weights were normal, suggesting that proliferation of decidual cells and trophoblasts, both of which express the CSF-1 receptor, may not be solely regulated by CSF-1. Histochemical staining for F4 80 antigen was used to identify macrophages in the uterus and placenta. Uterine macrophages could not be detected in virgin op op mice although they were abundant in+ op uteri. Interestingly, macrophages could be detected in op op uteri as uncharacteristically rounded cells in early gestation, however, they were not maintained and no macrophages were apparent beyond Day 14 of pregnancy in op op mice. Further studies in the osteopetrotic mouse will be useful in delineating those functions required for pregnancy that are regulated by CSF-1.