A group of 105 breeders and 36 unweaned guinea pigs was tested to determine the extent of specific fecal excretion and the proportion of serum reactors in a population naturally infected with two types of B. paratyphi. The second, more recent type of organism was isolated from the feces of 3 breeders and 3 young. No carriers of the first type were detected. 86 per cent of the breeders and 72 per cent of the unweaned guinea pigs agglutinated the second type of B. paratyphi in dilutions ranging from 1:10 through 1:640. 13 per cent of the breeders and 28 per cent of the unweaned young agglutinated the initial type. There was a cross or double agglutination in most instances. The serological findings roughly reflected the distribution of the two types as indicated by the mortality rate of the population at large.

50 breeders selected on the basis of agglutination and fecal examination and therefore supposedly free from infection were segregated and kept under close observation. Both types of B. paratyphi subsequently appeared in the group.

During this time carriers were discovered by others in the Department among the stock guinea pigs used for other experiments in that cultures of the entire spleen were positive in perhaps 5 to 10 per cent of the guinea pigs so used.

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