Freshly prepared mixtures of spotted fever virus and immune rabbit serum in neutral or superneutral proportions confer complete immunity on guinea pigs.

The mixtures undergo a considerable loss in immunizing power when heated to 60°C. for 20 minutes, but are still capable, if used in sufficient quantity, of conferring a degree of immunity on the vaccinated animal such that a subsequent experimental infection is rendered less severe and non-fatal.

Unheated mixtures which had been preserved in the refrigerator at 4°C. for a period of 32 days still retained a certain degree of immunizing property.

The virus alone, or mixed with normal rabbit serum, when allowed to die out by prolonged preservation at refrigerator temperature, or when killed either by heating at 60°C. for 20 minutes or by chemicals (chloroform, ether, xylene) does not induce immunity in guinea pigs.

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