It is thought that glucocorticosteroids impair the clearance of immune complexes by the mononuclear phagocyte system (MPS). We studied the effect of a five day course of prednisone (1 mg/kg body weight per day) on MPS function in 10 healthy volunteers, using soluble radiolabeled aggregates of human immunoglobulin G as a probe. MPS function was assessed before steroid treatment, and again 24 hours after the last dose of prednisone. Elimination kinetics and the uptake of the immunoglobulin aggregates by liver and spleen did not change after prednisone treatment. This suggests that, in contrast to general belief, high doses of glucocorticosteroids have no direct, acute effect on MPS function.