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The expression of incompatible A carbohydrate antigens in some adenocarcinomas may provide an explanation for the generally observed lower incidence of adenocarcinoma among types O and B versus type A individuals. The chemistry and genetic basis of incompatible A expression is largely unknown. Here, we have screened 31 cases of gastric tumors of phenotype O for the expression of blood group A gene-defined glycosyltransferase by immunohistology on frozen sections using newly developed monoclonal antibodies to the transferases. Three cases were positive, and transferase expression was confirmed by enzyme analysis of extracts from the specimens. Blood group A carbohydrate antigens were also identified immunohistologically in these three cases as well as in five other cases. Thin-layer chromatography immunostaining analysis of glycolipid extracts from the three cases did not confirm the chemical presence of A antigen. The ABO genotype of all patients was found to be OO, showing that all carried O alleles with a structural defect at nucleotide position 261 leading to a shift in the reading frame. The data suggest that incompatible A antigen expression is a result of transferase expression derived from the ABO genes.