Colonies that contain granulopoietic and erythropoietic cells can be grown in specimens of human bone marrow, peripheral blood, and cord blood. Growth of these colonies is promoted by media conditioned by leukocytes in the presence of phytohemagglutinin (PHA-LCM) and the addition of erythropoietin on days 4 or 5 to the cultures. Sedimentation velocity profiles for these granuloerythrocytic colonies suggest their origin from single cells (CFU-G/E) rather than from doublets or clumps. This hypothesis is supported by cocultivation of male and female specimens. Cells in granuloerythrocytic colonies that developed in such mixing experiments were either uniformly female by Y- chromatin analysis or contained Y-chromatin bodies in both the granulocytic and erythroid cells.