Using a new functional approach for the study of lymphomas and leukemias in which immunologic and cytochemical techniques were employed, we found a consistent surface immunoglobulin pattern of the γ-, κ-, λ-type on cells from poorly differentiated (acute) and well-differentiated (chronic) granulocytic leukemias. This pattern was also found on nonneoplastic granulocytes from patients with leukemoid reactions as well as on granulocytes from normal individuals. These findings suggested that both leukemia cells and nonneoplastic granulocytes had IgG bound to the cell surface by an Fc receptor. This binding of IgG by granulocytes was not tumors-pecific and appeared to correlate both with the degree of differentiation and possibly with the degree of activation of the granulocytes. In addition to raising the basic question of its functional significance, these findings offered an approach for distinction of poorly differentiated granulocytic leukemia from lymphomatous processes.