A formulation of oral vaccine consisting of Vibrio cholerae lipopolysaccharides (LPS), cell-bound haemagglutinin (CHA) and procholeragenoid (P), namely vaccine A, was compared with another formulation, vaccine B, prepared from killed whole vibrios plus procholeragenoid on their immunogenicity and reactogenicity in Thai male volunteers. Volunteers were randomly allocated into three groups. The first two groups received orally three doses of vaccines A and B, respectively at 14-day intervals. Volunteers in group 3 were controls and received orally 100 ml 5% (w/v) NaHCO3 also at 14-day intervals. Serum samples were collected from all volunteers before each immunization. Intestinal lavage was performed 3 to 7 days before the first dose of vaccine or placebo and 7, 21 and 45 days after the last dose. Serum vibriocidal antibodies were determined and class-specific, antigen-specific antibodies of all serum and lavage samples were assessed by indirect enzyme-linked immunosorbent assay (ELISA) using purified LPS, CHA and cholera toxin (CT) as antigens. Diarrhoea occurred in 10 and 40% of the vaccinees ingesting the vaccines A and B, respectively. The immunogenicity of the vaccine B in terms of seroconversion for vibriocidal antibodies and anti-LPS was higher than the vaccine A. Both vaccines had equal immunogenicity concerning serum anti-CT, while the vaccine A was slightly better than the vaccine B on serum anti-CHA response. The immunogenicity of the two vaccines in evoking intestinal responses was different from the systemic one.(ABSTRACT TRUNCATED AT 250 WORDS)