To understand factors that modulate the amount and removal of antigen found in the circulation of schistosome-infected mice, a specific schistosome antigen, GASP, found in the circulation of infected animals, was radiolabeled with Bolton-Hunter reagent and injected into infected and control mice. The rates of clearance, levels of antigen binding, and the fate of the injected antigen were determined. Within 1 to 3 wk after injection, infected animals developed enhanced antigen clearance that paralleled the increase in antibody to GASP. Lightly infected animals showed faster clearance rates and increased antibody levels compared with heavily infected mice. The level of antibody in the sera, irrespective of the duration or intensity of infection, correlated with enhanced clearance. Heavily infected animals removed preformed immune complexes as rapidly as lightly infected mice. This indicates that the relative inability to clear antigen was not due to blockade of Kupffer cells. Circulating GASP antigen, determined by countercurrent electrophoresis, was detected in heavily infected animals in the presence of increased antibody, suggesting that circulating GASP was in immune complexes. Both free and complexed antigen were primarily cleared by the liver and then excreted in the urine as low m.w. nonantigenic products. With sensitive radioimmunoassays, GASP could not be detected in infected human or mouse urine.