[Acquired B antigen: a new case].

A 86 year old woman, in good health but "Escherichia coli" urinary infection, was found to have an abnormality in blood group determination. 3 years ago, blood group of this patient was group A and now, precise determination shows the existence of an acquired B antigen. Disappearance of this acquired B Ag was obtained in 3 months after the urinary infection's treatment. The most cases of acquired B Ag result from neoplasic diseases or infection with enterobacteriacea. Physiopathology of acquired B Ag is secondary to the action of a deacetylase enzyme produced by enterobacteriacea, on group A1 erythrocytes.