We have proposed that the deposition in vivo of anti-brush border antibodies on proximal tubule cells in Heymann nephritis stimulates those cells to divide. To evaluate that hypothesis, we have investigated the temporal relationship between antibody deposition and kidney cell proliferation, using autoradiography to detect dividing cells in rats with Heymann nephritis and in age-matched controls treated with Freund's adjuvant alone. To assess the possible stimulation of proximal tubule cell proliferation by factors associated with proteinuria and/or nephrotic syndrome, kidney cell proliferation was measured in rats with chronic serum sickness glomerulonephritis. Proteinuric rats with chronic serum sickness also served as recipients of anti-brush border antibodies in passive transfer experiments. Cell division rates were not altered by adjuvant treatment or ageing. In both active Heymann nephritis and passive transfer experiments, a highly elevated stimulation of 3H-thymidine incorporation, reflecting mitotic activity, was detected in the proximal tubule epithelium immediately following the deposition of antibodies on the brush border. Significant enhancement of cell division was not noted in other nephron segments. A much smaller increase in proximal tubule cell proliferation accompanied proteinuria in chronic serum sickness. A similar small elevation compared to controls was also detected in late stages of Heymann nephritis when the proximal tubules were free of immunoglobulin deposits. It appears that the reaction of divalent antibodies with plasma membrane antigens can produce proliferative pathology of the proximal tubule epithelium. Furthermore, a significant, if less dramatic, enhancement of cell proliferation may be secondary to proteinuria and/or other manifestations of the nephrotic syndrome.