1. The blood of guinea pigs convalescent from Old World and New World typhus infection develops agglutinating properties for the tunica and rat Rickettsiae of the New World diseases and for the louse Rickettsia of the Old World disease.
2. The two microorganisms are closely related, though probably not identical.
3. Human convalescents of both varieties of typhus develop agglutinins for both types of Rickettsiae. Such Rickettsia-agglutinating properties are parallel with the Weil-Felix reaction in the human sera.
4. Rabbits immunized with Weigl louse vaccines develop agglutinins for our X-ray rat vaccines and vice versa. In both cases the rabbit sera develop agglutinins for Proteus X-19.
5. These experiments furnish a further and, we believe, conclusive proof of the etiological rôle, in New World typhus fever, of the Rickettsia bodies first seen in the tunica cells of inoculated guinea pigs by Mooser, and obtained in massive amounts by ourselves.
6. The serum reactions also provide a further logical basis for experiments in prophylactic vaccination with these Rickettsiae.