In a passive protection procedure in which the ed50 values of Bordetella pertussis 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.
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