The 1966-67 cholera vaccine field trials in East Pakistan tested 1- and 2-dose schedules of a commercial cholera vaccine in 40 000 children aged 3 months to 14 years. Randomsample serological surveys, made prior to the inoculations and 3 months and 6 months after the inoculations, demonstrated that there was a rise in the vibriocidal titres of the vaccinated children during the first 3 months after inoculation and a subsequent fall by the end of the second 3 months. The antibody response to 2 doses of cholera vaccine was better than the response to a single dose in children under 5 years of age. In children aged 5-14 years, the antibody response was similar for both inoculation schedules. Since the majority of the older children had vibriocidal antibodies before inoculation, the data suggest that the single dose acted as a booster, and this effect was not enhanced by a second inoculation.Serological studies of the hospitalized cholera patients indicated that the majority had low vibriocidal titres on admission to hospital. By comparing the distribution of admission titres of the hospitalized patients with the distribution of titres found in the population survey, it was possible to demonstrate a progressive reduction in the cholera case rate for the population with high levels of vibriocidal antibody.The sample surveys from the control population revealed a rise in titre following the peak of the cholera season, and a fall 3 months later. The data suggest that the rate of infection with V. cholerae for the 10 000 children in the control group during the cholera season may have been as high as 27%, while the clinical case rate was only 0.26%.