Anti-A and anti-K have been found in the serum of a 20-day-old child who had not been transfused but who was acutely ill with E. coli enterocolitis. Both antibodies are IgM proteins. The mother's serum does not contain either antibody and the anti-A and anti-K in the infant's serum are not of maternal origin. Both parents and the child are of the Kell phenotype K-k+. Stool cultures made from the child yielded E. coli O 125:B15, an uncommon B-variant pathogenic coliform. Cell-free preparations made from broth cultures of this organism have strong specific inhibitory activity against IgM anti-A and anti-K, and both antigens have been identified on the bacterial cells. At age 3 months the child had made a clinical recovery, stool cultures showed no pathogenic coliforms, and anti-A and anti-K were no longer detectable in her serum. These data indicate that absorption of metabolites with A-like and K-like activity produced by a pathogenic coliform in the intestinal tract were responsible for the appearance of apparent naturally occurring anti-A and anti-K in the child's serum.