Cells from CBA fetal mouse liver formed pure or mixed erythroid colonies in semisolid agarculture after stimulation by medium conditioned by pokeweed mitogen-stimulated mouse spleen cells. In general shape, the erythroid colonies resembled typical 7-day single or multiple (burst) colonies. However one-third to one-half contained, in addition to erythroid cells, macrophages and neutrophils and, less commonly, megakaryocytes or eosinophils. Culture of micro manipulated single colony-forming cells showed these erythroid colonies to be clones. Colony-forming cells declined in frequency with advancing fetal age, but low numbers were detectable in adult bone marrow. Assays of spleen conditioned medium in polycythemic mice failed to detect erythropoietin; the cloning system may detect a fetal type of erythropoietin-independent, erythropoietic cell since few were detected in adult marrow.