Interest in anti-Thomsen-Friedenreich (T) antibodies has increased because of their significance in detection of and their possible interaction with human adenocarcinoma. The origin of anti-T, which all humans possess, has not been ascertained. We determined here that anti-T and -Tn agglutinins could readily be induced via the physiological intestinal route by an enteric bacterium, E. coli O86, which possesses T and Tn activities. One dose of live E. coli O86 given in the drinking water to germfree chicks, who had no anti-T and -Tn antibodies, resulted, in all birds, in formation of saline agglutinating anti-T and -Tn antibodies as well as those detectable only by indirect agglutination. Antibody specificity was confirmed by adsorption on and elution from homologous human erythrocytes and for anti-T also by haemagglutination inhibition. In contrast, control chicks raised under ordinary conditions did have anti-T and -Tn prior to feeding E. coli O86. In humans, six diarrhoeic and five healthy infants and the majority of 13 adults investigated were fed killed rather than live E. coli O86. All infants, but one, suffering from diarrhoea showed a significant increase (greater than or equal to 4-fold) in anti-T and/or anti-Tn antibodies; in some, these antibodies were elicited de novo. All four adults with intestinal lesions had a significant increase of anti-T and/or -Tn subsequent to ingestion of E. coli O86, as did five of nine healthy adults, but to a lesser extent. These findings support the immune nature of demonstrable levels of anti-T and -Tn.