Sera from an immunogenicity study of killed oral cholera vaccine was tested in high and low binding Elisa plates. High binding plates yielded 5 to 7-fold higher prevaccination IgG antitoxin titres and about 3-fold higher post-vaccine antitoxin titres, but lower cero-conversion rates (39% for high and 68% for low binding plates) when they were compared with low binding plates, because the pre-vaccine titres were relatively higher than the post-vaccine titres. The high titre serum, being used as a standard control serum in each ELISA plate, did not detect the problem. We suggest using lowbinding plates when carryng out ELlSA assays for cholera antitoxin antibodies.