Two- to three-week-old mice homozygous for the recessive oc gene had negligible numbers of marrow cells but possessed no significant spleno- and hepatomegaly. They also maintained normal numbers of blood cells except for monocytes, which were significantly lower. Additionally, they had reduced numbers of total cells and resident macrophages in the peritoneum, as determined by cell counts in the peritoneal lavage fluid. The frequency of spleen colony-forming units (CFU-S) in the spleens of oc/oc mice was the same as that in the spleens of normal littermate control mice. These oc/oc CFU-S showed essentially similar differentiation patterns as CFU-S of control mice. Also, a few CFU-S could be detected in livers of oc/oc mice. On the other hand, the frequency of cells that formed macrophage colonies in a four-day liquid-culture system in the presence of colony-stimulating activity was significantly reduced in oc/oc mice and abnormalities were observed in the formation of the adherent (stromal) layers by oc/oc spleen cells in liquid cultures. Numbers of fibroblastoid cell colonies in these layers were reduced and, moreover, cultures demonstrated a marked decrease in the number of macrophages both within and outside the fibroblastoid cell colonies. Transplants of spleen and thymus cells of oc/oc mice into lethally irradiated +/? recipients induced oc/oc-like lesions. They included peritoneal macrophage deficiency, marrow deficiency, as well as hepatosplenomegaly. This suggests a hemopoietic stem cell and not microenvironmental defect in this particular type of osteopetrosis. The murine mutant characterized in this study may be useful in studies of cellular interactions during blood and bone formation and in studies of the mononuclear phagocyte system.