1. The results of the Weil-Felix reactions of sera from rabbits and monkeys inoculated with human virus or virus of laboratory strains of rural typhus, urban typhus or the tsutsugamushi disease are reported and discussed; as also are corresponding results in rabbits inoculated with the virus of two strains of tropical typhus recovered from wild rats.
2. Instances are given of a change in type of antibody response; in particular, an experience is described in which the agglutinogenic properties of a virus of a tropical typhus strain underwent a temporary change, while the immunogenic properties remained unchanged.