Macrophage, neutrophil, and eosinophil colony cells from bone marrow culture in semisolid agar medium were studied for membrane C3 and IgG receptors. The capacity of these cells to bind either erythrocytes-19S antibody-complement (EAC) or erythrocyte-7S antibody (EA7S) complexes was measured using the rosette method. Whereas macrophage and neutrophil colony cells showed receptors for both C3 and IgG, eosinophil colony cells appear to bear only IgG receptors. Studies correlating colony age and the presence of receptors showed that 60 to 70% of the cells from 3-day-old macrophage colonies were reactive for EAC and EA7S contrasting with 80 to 90% of the cells from 6- to 12-day-old colonies. Neutrophils behaved somewhat differently: EAC and EA7S reactive cells were seen in colonies after 4 or 5 days in culture and comprised only 50 to 60% of the colony population. Eosinophilic colonies showed 50 to 60% EA7S reactive cells after 6 to 7 days in culture, but no EAC reactive cells were found among these colonies at any time. The characteristics and properties of the receptors detected on colony cells were similar to those on macrophages and neutrophils from normal peritoneal fluid or bone marrow. Most macrophage colony cells were actively phagocytic whereas neutrophils and eosinophilic colony cells failed to show phagocytosis under the same conditions.