Mononuclear phagocytic function was studied using the Fc-receptor test in 24 patients who underwent splenectomy, ten of whom underwent splenic autotransplantation. All patients undergoing autotransplantation had mononuclear phagocyte system (MPS) activity at the transplantation sites. In eight of the 14 patients who did not undergo autotransplantation there was also scintigraphic MPS activity indicative of ectopic splenic tissue. Although the Fc-receptor test showed delayed and monoexponential blood clearance in all patients after splenectomy, there were no significant differences between the patient groups. Autotransplantation of small amounts of splenic tissue after splenectomy provides some MPS activity but is inadequate for blood clearance.