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Streptomycin-dependent Escherichia coliserotypes O 111, O 55 and O 26 were used to vaccinate young patas monkeys against these organisms by the oral route. Five administrations of 1010cells in 3-day intervals gave almost full protection to the animals against challenge with 9 to 11 minimal infective doses of homologous virulent organisms for at least 1 month, whether the three serotypes were administered singly or simultaneously.