IT HAS BEEN well established that blood group antigens, substances A and B, are genetically determined and develop during intrauterine life.1,2 In contrast, the blood group antibodies, the so-called "natural" isohemagglutinins, are believed by most authors to appear first, three to six months after birth.3,4 Whether these isohemagglutinins are inborn or the result of an antigenic stimulus has been a subject of experimentation and long debate. The theory that isohemagglutinin production takes place after birth from exposure to group A and B antigens found normally in ingested food and in the developing intestinal flora has many proponents.5-7 However, the presence of isohemagglutinins in neonatal sera8 has been demonstrated, and it is thought that these are maternal in origin and transferred through the placenta.4,9 Nevertheless, in recent years it has been well documented that the fetus produces small amounts of γM-globulins10 and is capable of