Crescentic glomerulonephritis in man, irrespective of aetiology, indicates severe glomerular damage and carries a poor prognosis. The importance of the presence of macrophages in the development of glomerular crescents is well established, but the contribution of cellular proliferation in both macrophages and epithelial cells has received little attention. The purpose of this investigation was to develop a model of crescentic glomerulonephritis in mice, which will be suitable for cell kinetic studies. Preliminary studies are described, involving immunization with rabbit anti-mouse glomerular basement membrane (GBM) antiserum after pre-immunization with rabbit immunoglobulin G, both with and without treatment with a fibrinolytic inhibitor, Cyklokapron. Extensive glomerular damage, with crescent formation, was induced with four daily intravenous injections of rabbit anti-mouse GBM antiserum, after pre-immunization with rabbit IgG. The effect was shown to be dose dependent and was seen within 3 days of the last injection. Linear deposits of rabbit IgG were detected in all glomeruli by immunofluorescence and both linear and granular deposits of mouse IgG were also found. There was a significant increase in glomerular size and in the numbers of cells in Bowman's capsule (P less than 0.01), and increases in mitotic indices to 0.8 per cent were seen by day 13.