Serial renal biopsies for glomerular culture, histochemical staining for β-glucuronidase, electron microscopy (EM) and light microscopy, were used to study macrophage involvement in experimental chronic immune complex (IC) glomerulonephritis (GN) induced in rabbits by daily intravenous injections of bovine serum albumin (BSA). In the 26 animals studied, proliferative GN of variable severity was induced, with mild disease in 5 animals, moderate proliferation in 15 and crescentic GN in 6. Macrophages first appeared in glomerular culture outgrowths during the 2nd and 3rd weeks coincident with the onset of proteinuria and rising serum creatinine concentration. Large numbers of macrophages (in excess of 20 per glomerulus) were seen by the 5th weeks and persisted to the 9th week. The number of macrophages in outgrowths was not significantly greater in animals with crescentic disease. EM demonstrated macrophages in capillary loops, and in glomeruli with crescents, macrophages could be seen in the urinary space. Histochemical staining for β-glucuronidase also demonstrated macrophages in the glomerular tuft and in crescents when present. These results indicate that macrophages constitute a considerable proportion of the glomerular hypercellularity seen in chronic IC glomerulonephritis.