Antigens of porcine lung were prepared by homogenization and ultrasonic treatment of lung tissue. The antigens were digested by collagenase and trypsin and chromatographed on Sephadex G 200. After immunization of rabbits with certain fractions of the chromatographed material, antibodies could be obtained in rabbits which fixed in vitro and in vivo in the glomerular mesangium of rats, hogs and the human, but not in the basement membrane of glomeruli of these species. Following intravenous injection into rats, the antibodies could be observed in the mesangial area without an apparent loss of antigenicity for 55 days. While 5 days after the application of 20 mg of the anti-mesangial IgG-preparation, there were no glomerular changes histologically demonstrable, after 55 days small amounts of rat-IgG could be demonstrated histochemically within the mesangium. Deposits of rat-complement (C3) could not be demonstrated with certainty. Severe morphological lesions were absent. Rats injected with the same doses of antibody and simultaneously immunized with rabbit-IgG showed a substantially greater deposit of autologous rat-IgG and rat-complement within the mesangial area after 55 days. By histological examination focal and segmental scleroses of the mesangium were determined. A significant pathological proteinuria did not occur. The present model constitutes a new possibility for studying the function of the glomerular mesangium.