Cellulose acetate membranes (CAM) placed in the peritoneal cavity of mice develop a macrophage layer capable of supporting in vivo hematopoietic colonies from intraperitoneally injected bone marrow cells. Modifications allowing for routine morphologic identification of colonies showed that both erythrocytic (E) and granulocytic (G) colonies occur with a consistent E:G ratio of 0.19 +/- 0.037. Stimulating recipients by bleeding or phenylhydrazine injection did not produce a significant change in the total number of colonies and a reduction in granulocytic colonies so that the E:G ratio significnatly increased. Hypertransfusion of donor animals had no effect on the number of erythroid colonies that grew on CAM of average recipients. The total colony-forming ability of bone marrow cells from genetically anemic W/WV mice was found not to differ from that of normal +/+ littermates; however, the E:G ratio of W/WV marrow in bled recipients was significantly lower (p less than 0.01) then that of +/+ marrow. These studies suggest that a CAM system supports an erythroid progenitor which is not affected by hypotransfusion of the donor animal, yet is dependent upon erythropoietin for colony formation, and that it is defective in the W/WV mouse.