Using aggregates of IgG (AIgG) obtained by heat aggregation of a 16 g% human IgG solution, we sought a method for measuring the function of the mononuclear phagocyte system with a probe that bears more resemblance to soluble immune complexes than erythrocytes coated with anti-rhesus IgG (EIgG). It was found that intravenous administration of 10 micrograms AIgG/kg body weight did not cause any detectable side effects in chimpanzees. In nine healthy volunteers, a dose of 10 micrograms AIgG/kg body weight was used without any adverse reactions. AIgG is cleared from the human circulation with a t1/2 of 26 +/- 8 min (mean +/- SD). The site of clearance is predominantly the liver, as shown by an average liver spleen uptake ratio of 230:100. In whole blood obtained from the volunteers, it was found that erythrocytes bound significant amounts of AIgG, suggesting that CR1 on erythrocytes is involved in the clearance of complement activating immune complexes in humans. In five of the volunteers, clearance studies with EIgG had been done in a previous study. EIgG was cleared from the circulation with a t1/2 of 30 +/- 6.2 min (mean +/- SD). The predominant site of clearance of EIgG was the spleen. These data indicate that sensitized red blood cells are cleared from the circulation differently from soluble IgG aggregates.