Recent studies have suggested that crescents are primarily of monocytic origin and that epithelial cells are a minor factor in their composition. Frozen sections of renal biopsies from 11 cases of crescentic glomerulonephritis (CGN) and 5 controls (2 acute interstitial nephritis, 1 focal glomerulosclerosis, 1 benign recurrent hematuria, 1 normal kidney) were stained for intracellular cytokeratin (CK) with a mouse monoclonal anti-CK antiserum (PKK1) and nonspecific esterase (NSE) activity. Indirect immunofluorescence with PKK1 antiserum showed that in all biopsies there was positive staining of collecting duct and proximal and distal tubular epithelium but no reactions in blood vessels or interstitium. In control case glomeruli there was no staining of the tuft, including the visceral epithelium. In all cases some parietal epithelium was CK-positive. In 4 CGN biopsies the majority of the crescents showed cytoplasmic staining for CK in more than 50% of the crescent cells. In 2 cases most crescents contained between 10-50% CK-positive cells, whereas in 5 biopsies little or no CK was present in the majority of crescents. In all but one CGN case the majority of crescents contained fewer than 30% NSE-positive cells (monocytes). Electron microscopy demonstrated intermediate filaments in many crescent cells and scattered desmosomes within crescents. The results indicate that epithelial cells, probably of parietal epithelial origin, contribute significantly to crescent formation.