A method was developed for the sensitization of ovine erythrocytes with a purified enterotoxin from Vibrio cholerae. Sensitized cells were used for the titration of serum antibody by passive hemagglutination and in a hemolytic plaque assay for both IgM and IgG antibody-secreting cells. Inhibition experiments with various antigens of V. cholerae indicated that the toxin, whether unheated or heat-inactivated, significantly reduced the expected antitoxic plaque-forming cell response, whereas a lipopolysaccharide-rich extract from homologous vibiros was not inhibitory.