In a randomised, double-blind, controlled field trial in Santiago, Chile, 81,621 schoolchildren aged 5-19 years received three doses, within a week, of attenuated Salmonella typhi oral vaccine Ty21a in enteric-coated capsules or in a new liquid suspension, or placebo. Over 36 months of surveillance, the liquid formulation (76.9% vaccine efficacy) was significantly superior to the enteric-coated capsules (33.2% vaccine efficacy). The liquid formulation protected young children (5-9 years) (efficacy 82.3%) as well as older children (efficacy 69.3%), whereas the capsules significantly protected only older children. The liquid suspension was easy to prepare by mixing lyophilised vaccine with buffer in water and was easily administered, even to the youngest children. Thus, the liquid formulation of Ty21a is preferable to enteric-coated capsules.