Macrophage uptake and presentation of thyroglobulin (TG) were investigated in rats with experimental autoimmune thyroiditis induced by immunization with TG and adjuvant. Peritoneal macrophages were shown to take up radiolabelled TG and this did not depend on the presence of thyroiditis in the animal or on the strain of rat used. Cell-associated antigen was largely unaffected by trypsin. Macrophages primed in vitro with TG produced a rise in the circulating TG autoantibody levels of convalescent animals when given intravenously (unlike free antigen), but did not induce disease in virgin animals. These findings show that macrophages can bind and present TG and suggest that this process may perpetuate otherwise transient autoimmune reactions.