Mouse peritoneal macrophages and macrophages derived from normal bone marrow progenitors in vitro possess surface receptors whose predominant specificity is directed toward the IgG2a subclass of IgG moleclues. Myelomonocytic leukemic cells also possess such IgG receptors but in an abnormally low frequency and without demonstrable subclass specificity. Within the leukemic population, monoblasts are distinguished from myeloblasts by their greater adhesiveness to glass, higher frequency of cells with surface receptors for IgG, and by erythrophagocytosis. Leukemic monoblasts, myeloblasts, and their morphologically mature progeny demonstrate impaired phagocytic ability. A concept of the development of surface receptors for IgG in the monocyte-macrophage cell line is presented.