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In a strictly controlled field trial among schoolchildren a study was made of the protective capacity of single doses of 2 typhoid vaccines: a heat-killed broth-grown sorbed dried vaccine made in the USSR (vaccine G-66) and an acetone-killed agar-grown dried vaccine (vaccine K-66) made in Yugoslavia. The latter vaccine was supplied by the World Health Organization.A single dose of these preparations exerted a statistically significant protective effect during the first 10 months of observation. The protection afforded by the acetone-killed and heat-killed sorbed vaccines was similar in duration and intensity to that resulting from single doses of other types of typhoid vaccines that had been evaluated earlier in the USSR. The data obtained confirm once again that successful immunization against typhoid is possible with a single dose of vaccine.The absence of any substantial difference in morbidity between the groups concerned indicates that the heat-killed broth-grown and acetone-killed agar-grown vaccines exert a roughly similar protective effect.