Recent studies in experimental crescentic glomerulonephritis, using the technique of glomerular culture, have shown that the macrophage is a major cell type present within the glomeruli and developing crescents. It has been suggested that their accumulation is a consequence of glomerular fibrin deposition. The effect of defibrination with ancrod on the cellular events occurring in experimental crescentic glomerulonephritis in the rabbit was therefore assessed in this disease using the techniques of culture of isolated glomeruli, electronmicroscopy or renal tissue, and light microscopy. Defibrinated animals developed only minimal renal impairment, virtually no fibrin deposition in Bowman's Space and only a mild degree of crescent formation, in contrast to the severe renal failure, fibrin deposition and crescent formation that occurred in the untreated animals. The culture of isolated glomeruli and electronmicroscopy of intact renal tissue demonstrated large numbers of macrophages within and emerging from glomeruli of both defibrinated and untreated animals. However, only in untreated animals were macrophages seen to migrate into Bowman's Space, phagocytose fibrin, transform into epithelioid cells and accumulate to form crescents. These studies suggest that fibrin deposition in Bowman's Space is the major stimulus to the macrophage migration from capillary loops and accumulation in Bowman's Space. However, fibrin deposition does not appear to be the stimulus to macrophage accumulation within capillary loops as this event was not affected by defibrination.