Rabbits injected with Freund's adjuvant, without any added antigen, developed antibodies against tubular brush border (BB) antigens of rat kidney, as demonstrated by the indirect immunofluorescence technique. After injection of the antibodies in rats, diffuse granular deposits of rabbit IgG, typical of passive Heymann nephritis, were detected along the glomerular capillary walls. The adjuvant-induced anti-BB antibodies thus resembled the immune anti-BB antibodies elicited in rabbits by immunization with isolated rat kidney BB membranes. Both the in vitro and in vivo binding of the adjuvant-induced antibodies could be inhibited by absorption with isolated rat kidney BB membranes. To characterize the BB antigens, radioactively labeled and solubilized BB membrane vesicles were precipitated with the adjuvant-induced and the immune anti-BB antibodies. Analysis of the immune precipitates by gel elecrophoresis showed polypeptides of more than 200, 130, and 80 kilodaltons as the major components of the precipitates. These antigens are major glycoproteins of the BB membranes and appear to be associated with the nephritogenic antigens of passive Heymann nephritis.