Erythrocytes from a patient with blood type of A1 became nongenetically reactive with A (anti-B) serum, following the development of gastric cancer. Transient polyagglutinability was also apparent. The in vitro acetylation of the erythrocyte antigens abolished both the acquired B antigen and the polyagglutination. Although incubation of the heterologous type A1 erythrocytes with the patient's serum did not produce B antigen, deacetylase activity in the serum seems to relate to the acquired B antigen. In this case, either ileus as a result of metastatic adenocarcinoma of the stomach, or urinary tract infections could be the cause of the acquired B antigen.