Monoclonal antibodies (MAbs) to specific keratin subtypes were prepared and characterized by immunoblotting and immunohistochemical assays on human cell cultures and normal and malignant human tissues. Chain-specific MAbs to keratin 7 (RCK 105, OV-TL 12/30) and keratin 18 (RGE 53, RCK 106, CK18-2), as well as broadly cross-reacting keratin MAbs (RCK 102, OV-TL 12/5) could be shown to react with different types of human epithelial tissues and were therefore tested for their usefulness in the differential diagnosis of carcinomas. The two broad-spectrum antibodies stained virtually all of the more than 350 carcinomas tested, especially when combined, and distinguished them from most nonepithelial tumors. The keratin 18 MAbs distinguished adenocarcinomas (which are keratin 18 positive) from most squamous cell carcinomas (which are generally keratin 18 negative). The MAbs to keratin 7 could be shown to recognize specific subtypes of adenocarcinoma and could, for example, distinguish between ovarian carcinomas (keratin 7 positive) and carcinomas of the gastrointestinal tract (keratin 7 negative), or between transitional cell carcinomas (keratin 7 positive) and prostate cancer (keratin 7 negative). In general, malignancies showed the expected keratin reactivity pattern as concluded from the keratin pattern of its cell of origin or its type of differentiation. The use of an extended series of malignancies did, however, also illustrate that exceptions to this rule exist. For example, certain antibodies to keratin 18 stained tumor areas in squamous cell carcinomas of the lung. Also a certain percentage of tumors, which generally showed no keratin 7 expression, were positive with RCK 105 or OV-TL 12/30. On the other hand, a certain percentage of tumors, which were generally positive for keratin 7, did not show a staining reaction with these MAbs. Furthermore subtle differences between reactivity patterns of different MAbs recognizing the same keratin protein were observed, both in the normal and malignant human tissues, indicating that specific keratin epitopes may be masked in certain tissues and that unmasking of such epitopes can occur with malignant progression. This phenomenon may be of some use in a further subtyping of carcinomas, especially those of the gastrointestinal tract. Despite these exceptional staining patterns, the keratin MAbs described above have proved to be useful tools in the characterization of epithelial tumors in routine histopathology and cytopathology, in which they add to a more refined diagnosis of (adeno)carcinomas.