Keratin polypeptides of the nonhair type, numbered 1 through 20 in the Moll catalog, are selectively expressed in normal and neoplastic tissues. Keratin 1 (K1), the highest-molecular-weight keratin (67 kd), was generally considered specific for keratinizing squamous epithelia. However, recent studies have shown that it is an integral component of the multiprotein kininogen receptor of endothelial cells. A library of formalin-fixed, paraffin-embedded tissue samples was evaluated immunohistochemically (avidin-biotin peroxidase complex method) for K1 expression using a specific monoclonal antibody (Novocastra clone 34betaB4). The study group included a wide variety of normal tissues and 541 tumors of epithelial or mesenchymal derivation. The specificity of the antibody to K1 was verified in normal epithelial tissues, where the staining was essentially limited to the epidermis and Hassal corpuscles of the thymus and focally to other squamous epithelia. Among carcinomas, it was essentially limited to keratinizing squamous carcinomas. It was also regularly found in endothelial cells of normal capillaries, veins, and arteries. Capillary, cavernous, and venous hemangiomas often had endothelia with K1 positivity. Among the malignant vascular tumors, epithelioid hemangioendotheliomas were consistently positive (8 of 8). However, angiosarcomas had more variable expression (59 of 81 were positive), with well-differentiated tumors generally having greater reactivity than poorly differentiated examples. Mesenchymal tumors with K1 expression included schwannomas (10 of 16), epithelioid sarcomas (26 of 37), and synovial sarcomas (19 of 68). In the last 2 tumor types, K1 reactivity was detected in both epithelioid and spindled neoplastic populations. In addition to its specificity for keratinizing squamous epithelia, K1 can be immunohistochemically detected in normal vascular endothelial cells and a spectrum of vascular tumors. However, its expression in poorly differentiated vascular tumors is variable, suggesting that this marker is poorly conserved in highly transformed endothelia. The unexpected K1 immunoreactivity in nonvascular soft tissue tumors, such as synovial sarcoma, epithelioid sarcoma, and schwannomas, requires further study.