This study was undertaken to analyze the mechanisms of immune complex formation in Heymann nephritis. We isolated two different RTE antigens by gel filtration and prepared rabbit antisera against these antigens. By the indirect immunofluorescence method using normal rat renal tissues, the 65,000 molecular weight antigen (= beta) was observed not only in RTE, but also in the glomerular epithelium, epithelium of the small intestine, liver and spleen. On the other hand, the 35,000 molecular weight antigen (= gamma) existed in RTE and epithelium of the small intestine. When rats were injected intravenously with rabbit antiserum against beta, glomerular depositions were observed within two hours. In rats injected with rabbit antiserum against gamma, no glomerular deposition was seen within 2 days, but fine granular depositions were observed after 6 days. When rat kidneys were perfused with rabbit antiserum against gamma in saline by a single pass method, no glomerular deposition was seen. However, in rat kidneys perfused with preformed soluble gamma-anti gamma IC, fine granular depositions along the capillary walls were seen soon after the perfusion. Further the antigen which was reacted with anti gamma antiserum was isolated from normal rat serum by immuno-affinity chromatography. These facts suggest that the mechanisms of IC formation may be due to not only in situ immune complex formation but also circulating immune complex deposition in Heymann nephritis.