A field trial of oral streptomycin-dependent mutant Shigella vaccines in five hyperendemic areas of Yugoslavia in 1969 confirmed the findings of earlier studies by demonstrating the effectiveness of these vaccines against dysentery. For the first time, a high degree of protection was demonstrated in children. The vaccines induced serotype-specific immunity against Shigella flexneri 1 and 2a and S. sonnei. Postvaccinal reactions were minor and consisted of vomiting or diarrhoea, or both, in a small proportion of children within several hours of the administration of the vaccine. These reactions, seen mainly after the first dose, were dose-dependent and could be decreased by reducing the number of live organisms. Reactions to subsequent doses were much fewer. Pretreatment with sodium bicarbonate was necessary. Under the conditions of this study, the vaccines proved to be stable with no evidence of reversion of the mutant strains to the virulent parent.