Human epithelial cells contain, intermediate-sized filaments formed by polypeptides related to epidermal alpha-keratin ("cytokeratins") which are expressed in different combinations in different epithelia. Using cytoskeletal proteins from human biopsies and autopsies we have examined, by two-dimensional gel electrophoresis and immunoblotting experiments, the cytokeratin polypeptide patterns of diverse primary and metastatic carcinomas and have compared them with those of corresponding normal epithelial tissues and cultured cells. Five groups of carcinoma cytokeratin patterns can be discriminated. (1) Cytokeratins typical of simple epithelia (polypeptides Nos. 7, 8, 18, 19) are expressed, in various combinations, by many adenocarcinomas, for example those of gastrointestinal tract. (2) Cytokeratins typical of stratified epithelia (Nos. 1, 5, 6, 10, 11, 14-17) are found, in various combinations, in squamous cell carcinomas of skin and tongue. (3) Complex patterns showing polypeptides Nos. 7, 8, 18, 19, and one basic component (No. 5 or 6) are detected in certain carcinomas of the respiratory tract and the breast. (4) Complex patterns containing cytokeratins widespread in stratified epithelia (Nos. 4-6, 14-17) as well as components Nos. 8 and 19 occur in diverse squamous cell carcinomas derived from non-cornified stratified epithelia, with or without additional small amounts of cytokeratin No. 18. (5) Patterns of unusually high complexity can be found in some rare tumors as is shown for a cloacogenic carcinoma. No significant qualitative changes of expression of cytokeratins were found when primary tumors and metastases were compared. When compared with cytokeratin patterns of normal epithelia, carcinomas of the first type usually display a high degree of relatedness to the tissue of origin. Other carcinomas do not express some of the cytokeratins present in the tissue of their origin and, vice versa, certain components which are minor or apparently absent in normal tissue are major cytokeratins in the corresponding tumor. These differences may be explained by cell type selection during carcinogenesis, but changes of expression during tumor development cannot be categorically excluded. The possibility of cell type heterogeneity within a given tumor is also discussed. Similarly complex patterns of cytokeratin polypeptides have been noted in certain cultured human carcinoma cell lines (e.g., A-431, RPMI 2650, Detroit 562, A-549) and can also be observed in cell clones. The possible value of analyses of cytokeratin patterns, by gel electrophoresis or specific monoclonal antibodies, in distinguishing different carcinomas by non-morphologic criteria is discussed.