The origin of glomerular crescents was investigated in an accelerated model of rabbit nephrotoxic nephritis. In rabbits immunised against sheep gamma-globulin, the administration of sheep nephrotoxic serum provoked cellular crescent formation affecting 30 to 90 per cent. of glomeruli at 6 days. The crescents were composed predominantly of cells with ultrastructural features of macrophages. In animals depleted of circulating leukocytes by whole body irradiation, with or without shielding of the kidney, crescent formation was inhibited despite severe glomerular damage and fibrin deposition in Bowman's space. These findings support the hypothesis that glomerular crescents develop principally from the emigration and accumulation of bone marrow-derived mononuclear cells.