It was investigated whether or not the human blood group isoantibodies A and B could be induced by immunogenic stimuli via natural routes with a kind of antigenic substance to which all humans are commonly exposed, or if the appearance of these antibodies is independent of antigenic stimuli as has long been believed. Escherichia coli O(86), which possess high human blood group B and faint A activity in vitro, were fed to healthy humans and those with intestinal disorders. 80% of the sick individuals of blood group O and A responded with a significant rise of anti-B antibodies which was frequently de novo in infants; significant increase of anti-A isoantibodies among blood group O individuals was less frequent. Over one-third of the healthy individuals also had a significant isoantibody increase. Intestinal lesions favor isoantibody stimulation by intestinal bacteria; this view was supported by the study of control infants. Persons of blood group A responded more frequently with anti-B and anti-E. coli O(86) antibody production than those of blood group O. Isoantibody increase was accompanied with antibody rise against E. coli O(86). Inhalation of E. coli O(86) or blood group AH(O)-specific hog mucin also evoked isoantibodies. The induced isoantibodies were specifically inhibited by small amounts of human blood group substances. E. coli O(86)-induced anti-blood group antibodies in germ-free chickens and preexisting blood group antibodies in ordinary chickens were neutralized by intravenous injection of E. coli O(86) lipopolysaccharide. This study demonstrates that human isoantibodies A and B are readily elicited via physiological routes, by blood group-active E. coli, provided the genetically determined apparatus of the host is responsive. Antibodies against a person's own blood group were not formed. Interpretation of these results permits some careful generalizations as to the origin of so-called natural antibodies.