The IgM monoclonal antibody G10 was raised against the human squamous cell carcinoma (SCC) cell line UM-SCC-1. In initial screening against cultured cells, G10 bound to 2 SCC lines (UM-SCC-1 and UM-SCC-13) and 1 pancreatic carcinoma line (UM-PAd-1) but not to cultured fibroblasts (WI-38), ovarian carcinoma cells (SK-OV-3), or malignant melanoma cells (SK-MEL-28 and MeWo). In subsequent tests against cultured cell lines, G10 gave positive reactions with 30 of 33 SCC lines but only 4 of 29 non-SCC lines. The non-SCC lines that bound G10 were UM-PAd-1, 2 transitional cell carcinoma lines (T24 and RT4), and 1 melanoma line (SK-MEL-22). When tested against cultures derived from normal skin or mucosa, G10 was reactive with the epitheloid squamous cells but not with the fibroblasts in each culture. The antigen defined by antibody G10 was stable to fixation with Formalin, and its distribution in tissue sections was examined with the use of immunoperoxidase assays. All SCC biopsy specimens examined in this way were reactive with antibody G10. In similar tests against sections of fixed normal tissues, G10 stained the superficial squamous cells of the epidermis and the basal and suprabasal layers of mucosal squamous epithelial cells from the esophagus. All layers of the laryngeal epithelium were positive. Endothelial cells and certain glandular cells were also positive for G10 binding. G10 agglutinated human red blood cells of all blood groups except those from individuals of the Bombay group (Oh) who lack the H blood group determinant. Against defined oligosaccharides, G10 bound strongly only to the monofucosyl H type 2 structure and was slightly cross-reactive with the synthetic difucosyl H type 2 or Y structure. These results are consistent with previous reports of blood group antigen tissue distribution and indicate that the H type 2 determinant is expressed by all or nearly all mucosal squamous cancers. Less frequent expression by cells of other tumor types may correlate with tissue-specific activation of the H gene-specified fucosyltransferase.