1. Guinea pigs can be actively immunized against European typhus fever with homologous formalinized Rickettsia tissue cultures, provided sufficient amounts are injected. The method is suggested for practical application in man.

2. Serovaccination against European typhus fever can be successfully applied to guinea pigs by a variety of methods, the simplest of which consists of the injection of mixtures of virulent defibrinated guinea pig blood and convalescent guinea pig serum taken from 3 to 5 days after defervescence. Similar results can be obtained with mixtures in which tissue culture virus, either with convalescent guinea pig serum or with antimurine horse serum, is used. There is no indication so far that such animals become carriers.

Possible application of these methods to typhus epidemics is discussed.

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