Epithelial cell membrane-bound blood group antigens A and B are lost in premalignant and malignant oral lesions. We now show that this loss in premalignant lesions is accompanied by accumulation of a blood group antigen precursor. The precursor structures were: type 2 chain H-antigen, Fuc alpha 1 leads to 2 Gal beta 1 leads to 4 GlcNAc-R (A and B precursor); and N-acetyllactosamine, Gal beta 1-4GlcNAc-R (H-precursor), in which Fuc is L-fucose, Gal is galactose, and GIcNAc is

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... ne. They were demonstrated in tissue sections by immunohistochemical staining techniques with monoclonal antibodies to H-antigen and N-acetyllactosamine. Precursors were found only on basal and suprabasal cells of normal mucosa. In all nine of ten premalignant lesions, the H-antigen was found on all cell membranes in the epithelium, in higher titers than in normal adjacent epithelium. Ten carcinomas were studied, and all showed an irregular distribution of H-antigen. N-Acetyllactosamine was not found in premalignant or malignant lesions. The accumulation of type 2 chain H-antigen in oral premalignant lesions may prove helpful in early diagnosis of epithelial cancer.