Immunogenicity of killed whole vibrio and B subunit oral cholera vaccines in American and Thai volunteers were analysed in terms of significant rise of antibody titre. Three doses of 2 x 10(11) killed vibrios and 5 mg of cholera toxin B subunit were given at two-week intervals. There were no differences in the percent of volunteers with significant rise of serum immunoglobulin G and secretory immunoglobulin A (sIgA) to cholera toxin. However, the percent with significant rises of serum antibody to whole cell V. cholerae Inaba measured by vibriocidal titre and serum immunoglobulin G, and secretory immunoglobulin A to lipopolysaccharide (LPS) measured by ELISA in American volunteers were significantly different from those in Thai volunteers (89% VS 45%, 68% VS 9% and 53% VS 0%, respectively) (p less than 0.05).