Guinea pigs can be immunized against Mexican typhus virus by peritoneal injections of formalinized Rickettsia material, provided sufficient amounts of the organisms are used. Our results in this respect are analogous to those of Spencer and Parker with carbolized virus of Rocky Mountain spotted fever.

The Rickettsia suspensions appear to possess considerable toxicity.

We do not wish to be misunderstood as implying that the results in guinea pigs offer anything more than a demonstration of the principle of active immunization with killed Rickettsiae. Application to man will have to be worked out, and preliminary to this, we are now attempting to apply the methods to a limited number of monkeys.

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