The oral-formalin inactivated whole cell enterotoxigenic Escherichia coli (ETEC) vaccine needs to be further tested in developing countries in order to determine the dose at which it will be safe and immunogenic for infants who are the target population for the vaccine. To determine the immunogenicity of reduced doses, studies were first carried out in children, 2-12 years of age (n = 60). The full, half or a quarter doses of the vaccine were comparable in immunogenicity with similar frequency of responses seen to the different antigens (P = NS). Following this result, a pilot study carried out in infants, 6-17 months of age (n = 50), showed that the frequency of episodes of vomiting was lowest when a quarter of the full dose was used. The infants however showed comparable immune responses to the half and quarter dose of vaccine that was tested (P = NS). Based on these results in the infants, a randomized double blind placebo-controlled Phase II study was carried out in 158 children, 6-17 months of age, where a quarter dose of the ETEC vaccine was tested. Adverse events of mild vomiting were seen in only 4% of vaccinees and in 2.5% of placebo recipients. The IgA-antibody secreting cell (ASC) responses to CFA/I (GM: 28.1 ASC/10(7) PBMC) and BS (GM: 55.7 ASC/10(7) PBMC) were elevated compared to placebo recipients (CFA/I-2.0; BS-4.8 ASC/10(7) PBMC) (P = 0.01 to < 0.001). The plasma-IgA antibody titers in vaccinees were also significantly elevated to CFA/I (GM-93.00), CS1 (GM-62.0), CS2 (GM-55.0), CS4 (GM-66.0) and BS (1057.0) compared to preimmune levels or responses or levels in placebo recipients (P < or = 0.05-0.001). This study thus demonstrates that reduced doses of the ETEC vaccine is immunogenic in children and infants as well as safe in infants down to 6 months of age.