Blood group typing of an aged patient suffering from ileus provided evidence for an acquired B. As a parameter independent of cell membrane molecules or secreted blood group substances, the nucleotide sequence polymorphism of A and B transferases was investigated. Restriction fragment length polymorphism of DNA chains amplified in a polymerase chain reaction from the coding region of the glycosyltransferase indicated that no gene for B transferase was present in the patient's genome. We conclude that the assessment of polymorphism of AB0 blood group transferase can be used as a marker independent of blood group molecules for confirming a suspected acquired B.