The relative contributions of glomerular epithelial cells, macrophages, and other cell types to the formation of cellular crescents characteristic of human rapidly progressive glomerulonephritis remain controversial. To identify and quantitate the cell types present during different stages of glomerular crescent formation, immunoperoxidase labelling of cryostat sections from renal biopsies with cellular (n = 9) or sclerosed (n = 3) crescents was performed using monoclonal antibodies to cell-specific antigens of leucocytes, epithelial cells, and other glomerular cell types. Fresh cellular crescents consisted of macrophages (34.5 +/- 7.0%; mean +/- SEM) plus lesser proportions of polymorphs (12.8 +/- 4.7%) and epithelial cells (10.4 +/- 1.5%). Sclerosed crescents contained fewer macrophages (5.1 +/- 1.0%), but similar proportions of polymorphs (11.1 +/- 2.9%) and epithelial cells (11.5 +/- 2.1%). Lymphocytes were not detected within crescents. Many of the remaining unlabelled cells morphologically resembled fibroblasts and expressed surface fibronectin, though fibroblast-specific cell markers were not available. These results show that macrophages and not epithelial cells constitute the major cell type within cellular crescents. Therapeutic manoeuvres directed against macrophages may, therefore, be of clinical value in the management of human crescentic rapidly progressive glomerulonephritis.