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Preston's description of Bordetella pertussis 353-Z as a culture possessing only factor 1 was confirmed by agglutination and agglutinin-adsorption tests. The LD50 values of this culture for mice inoculated by the intracerebral route were approximately the same as that of B. pertussis 18-323, the challenge culture in mouse potency tests of pertussis vaccine. In mouse protection tests, vaccine prepared with the factor 1 culture protected as well against challenge with B. pertussis 18-323, which has a broad antigenic pattern, as did vaccines made with cultures possessing factors 2, 3, 4, and 5 in addition to factor 1. When two groups of similarly immunized mice were challenged with 353-Z and 18-323, respectively, much lower ED50 values were obtained with the animals challenged with 353-Z. This was true whether a factor 1 vaccine or a vaccine with factors 1, 2, 3, 4, and 5 was tested.

In a passive protection procedure in which the ED50 values of Bordetellc/ pertuessis antisera were determined, groups of mice were given graded intraperitoneal doses of serum, followed the next day by intracerebral challenge with 100,000 organisms. Antiserum produced with B. pertussis culture 5373, serotype 1 3, protected mice against challenge with culture 18-323, serotype 1 2. 3, as effectively as did an antiserum produced with a serotype 1 2.3 culture. When two groups of mice similarly treated with pertussis immune serum were challenged with culture 353Z (serotype 1) and 18-323, respectively, much lower ED50 values were obtained with the animals challenged with 353Z. Passive protection tests with adsorbed antiserum gave equivocal results, suggesting that some of the adsorbing antigen remained in the serum and interfered with the tests. There was no evidence that serotype is related to protection.