The virus isolated by Derrick from cases of fever produces characteristic pathogenic effects on monkeys and mice. A well-defined febrile reaction, during which the blood is infective for guinea-pigs, follows subcutaneous inoculation of virus in the monkey. Mice inoculated intraperitoneally show enlargement of spleen and liver with characteristic histological changes. In sections and smears of infected mouse liver and spleen large numbers of rickettsial organisms are visible. These occur in relatively large intracytoplasmic microcolonies. Inoculation of mouse virus into guinea-pigs and monkeys gives a febrile reaction with a very short incubation period. Cross immunity tests indicate that mouse virus is identical with guinea-pig virus. The virus produces an inapparent infection in albino rats. The virus survives, but does not produce characteristic lesions on the chorio-allantois of the developing egg. The virus is filterable with difficulty through relatively permeable (0.7 mu) gradocol membranes. Agglutination of semi-purified rickettsial suspensions by immune human monkey and guinea-pig sera has been observed; this reaction appears to be specific.