The expression of intermediate filament proteins, particularly individual cytokeratins (CKs), vimentin, and glial filament protein, was immunohistochemically investigated using frozen sections and Carnoy-fixed, paraffin-embedded tissue from normal fetal and adult human kidneys as well as from pathologically altered kidneys. In fetal kidneys, the co-expression of CKs and vimentin was detected in the visceral and parietal epithelium of the glomerulus, the proximal tubules, the thin loops of Henle, and the collecting ducts. In contrast, in the tubules of normal adult kidneys, the presence of vimentin and CKs was nearly always mutually exclusive. While CKs 8 and 18 were present in all tubular epithelia, CKs 19 and 7 each exhibited a distinctive distribution pattern, there being a striking alteration between positive and negative segments and, not infrequently, intratubular heterogeneities. In certain segments, particular cell types (e.g., "plica cells," intercalated cells) could thus be recognized. In tubular epithelia altered by various injurious conditions, novel or enhanced expression of vimentin, CK 19 and CK 7, and, less frequently, CK 17 and glial filament protein was noted in certain segments. The increase in intermediate filament protein expression in altered (particularly proximal) tubules appeared to parallel the reduction in the degree of differentiation. Vimentin was never detected in distal tubules. The present results reveal a considerable similarity between the intermediate filament patterns in non-neoplastic proximal tubules of fetal and damaged kidney tissue and those in clear-cell and chromophilic renal cell carcinomas. They also serve to illustrate that the analysis of both fetal development and reactive cell changes may significantly contribute to our understanding of differentiation phenomena in malignant tumors.