In May 1963 a live vaccine prepared from streptomycin-dependent Shigella flexneri 2a was administered to 355 soldiers stationed in an area of Yugoslavia in which bacillary dysentery was hyperendemic. Five oral doses were given every third day. 382 unvaccinated soldiers from the same unit served as controls. Both groups were closely followed up until 16 September.Examination of 1637 rectal swabs from the vaccinated and control groups indicated that vaccination did not affect the carrier rate. The morbidity from shigellae of serotypes not contained in the vaccine was the same in the two groups; but Sh. flexneri 2a caused dysentery in 5.5% of the control group but in none of the vaccinated soldiers-a statistically significant difference.A similar vaccine, given in a much lower dose, was also tried among soldiers of another unit; the dose was insufficient to confer protection.The authors conclude that the results of these field trials justify further studies with live Shigella vaccine.