It has been shown previously that rat oral epithelia express antigens cross-reacting with antibodies against human blood group antigen B and its structural precursor, the H antigen (Type 2 chain). In the present study we investigated the expression of these antigens in malignant changes in the rat palate induced by a chemical carcinogen (4NQO). The H antigen, normally expressed on spinous cells in rats, was absent in malignant epithelium, whereas staining for the B antigen, normally expressed on basal cells, was variable. These changes are equivalent to those seen in human squamous cell carcinomas. The blood group antigen staining pattern in experimentally produced verrucous carcinomas showed an almost normal blood group antigen expression. This may have diagnostic significance. Localized areas of hyperplastic palatal epithelium with slight dysplasia revealed loss of H antigen and the presence of B antigen in suprabasal strata equivalent to the pattern seen in human premalignant epithelium. We conclude from these findings, that the rat model is well suited to study changes in cell surface carbohydrates during chemical carcinogenesis.