The relationship between asymptomatic fecal carriage of enterotoxigenic bacteria (ETB), diarrhea and serum immune response in children has not been systematically explored. We studied 40 babies between birth and two years with weekly health histories, weekly fecal samples and serum samples every six months and with diarrhea. Toxigenic bacteria and fecal toxin were identified by ELISA (cholera toxin) and Y-1 cell assay. Serum antitoxin was identified by ELISA (IgG and IgM) and toxin neutralization (Y-1 assay). Thirty-eight babies had fecal ETB 457 weeks; 36 of them had toxigenic E. coli (ETEC) 220 weeks. Twenty babies had 36 episodes of diarrhea within one week of isolation of ETB or demonstration of fecal toxin. Nineteen (53%) episodes in 11 (55%) babies were followed by an antibody response within one to three weeks. Sixty-one sera were positive. Forty-three (69%) of the positives were preceded within one month by toxin and diarrhea. Ten (17%) were preceded by diarrhea but no toxin; five were preceded by toxin but no diarrhea; three were preceded by neither. Twenty-four (64%) of the babies who carried ETB had a serologic response to toxin at some time during the observation period. Antibody responses followed identification of toxigenic E. coli in 17 instances, Klebsiella in four, Citrobacter in two, colony pools in four, and fecal water in 21. Asymptomatic carriage of enterobacteria which elaborated toxin immunologically and functionally similar to cholera toxin was common among these babies but serum antibody responses were significantly more common following toxin-related diarrhea.