A ganglioside enzyme-linked immunosorbent assay (ELISA) was used to study and attempt to differentiate between antitoxin responses in persons infected with either Vibrio cholerae or Escherichia coli producing heat-labile enterotoxin. In most cases (69%-94%), experimentally infected North Americans and naturally infected Bangladeshis responded to either infection with significant (greater than twofold) increases in serum antibody titer to both heat-labile enterotoxin and cholera toxin. In all but one instance, the response was higher to the homologous than to the heterologous toxin, and for the Americans the homologous antitoxin titers remained significantly higher for at least one year. Determination of levels of antibodies to purified subunits A and B of cholera toxin by an ELISA showed that V. cholerae infection in most instances induced a significant response to subunit B but rarely to subunit A. E. coli infection, on the other hand, induced only slight increases in antibody titer to either subunit.