The authors have prepared and studied three murine monoclonal antibodies that are reactive with antigens in brush border regions of proximal renal tubules of rats. Two of the antibodies, 14C1 and AG3, were derived from mice immunized with Fx1A and the third, 4H6, from a mouse immunized with isolated glomeruli obtained from rats with Heymann nephritis. Immunohistochemical, immunoelectron microscopic, and immunochemical studies showed that the three antibodies recognized different antigens. The antibody 14C1 recognized the previously described nephritogenic glycoprotein, gp 330, in microvillar brush border preparations, and reacted with material present on podocyte cell surfaces of normal rat kidneys, especially in coated pits, as well as with material in the glomerular deposits of rats with Heymann nephritis. The antibody 4H6 recognized a 110-kilodalton microvillar antigen and reacted with material in the glycocalyx of podocytes of normal glomeruli, but showed only equivocal reactivity with material in the deposits in Heymann nephritis. AG3 failed to immunoprecipitate a distinctive antigen in microvillar preparations and did not react with the glomeruli of normal rats or of rats with Heymann nephritis. All three antibodies reacted with epithelial cells in the intestine, epididymis, and placenta; 4H6 also reacted with thin loops of Henle as well as with endothelial cells or other cells in lung, lymphoid tissue, liver, and spleen. The results demonstrate that not all brush border antigens participate in Heymann nephritis and confirm that an antigen (gp 330) involved in the formation of glomerular deposits in Heymann nephritis is normally present on podocyte surfaces, especially in coated pits, but is not present in extracellular sites.