M acrophages were shown by the use of glomerular cell culture and morphologic techniques to be present in large numbers within the glomeruli of rabbits with acute serum sickness (AcSS) and in a passive model of the autologous phase of antiglomerular basement membrane (GBM) antibody-induced glomerulonephritis (PAGBMN). To determine the part played by these cells in the glomerular injury, animals were treated with a sheep anti-rabbit macrophage serum (AMS) or normal sheep serum (NSS). NSS administration had no effect on the development of either model of glomerulonephritis. The use of AMS reduced the number of circulating monocytes and prevented the accumulation of macrophages within glomeruli in both models (AcSS/NSS, mean 126/glomerulus, range 40-251; AcSS/AMS, mean 8, range 1-44; PAGBMN/NSS, mean 52, range 27-69; PAGBMN/AMS, mean 5, range 2-7). The AMS-treated rabbits had only minor histologic lesion and profound reduction in proteinuria (AcSS/NSS, mean 516 mg/24 h, range 200-991; AcSS/AMS, mean 41, range 3-161; PAGBMN/NSS, mean 335, range 55-975; PAGBMN/AMS, mean 10, range 2-24). Similar studies in the heterologous phase of glomerular injury induced by the same anti-GBM antibody revealed no effect of the AMS on this polymorphonuclear leukocyte-related phase of injury, demonstrating the selectivity of the antisera. Complement depletion, with cobra venom factor, did not affect the development of glomerulonephritis nor the accumulation of macrophages in either model. Inhibition of macrophage accumulation can largely prevent these forms of experimental glomerulonephritis, thereby implicating macrophages as mediators of glomerular injury and consequent proteinuria.