SUMMARYAerosol and subcutaneous vaccination with killed arthrospores and subcutaneous vaccination using a boivin-type fraction were compared for their efficacy in protecting rhesus monkeys against lethal aerosol challenge withC. immitis.Complete protection against lethal challenge was produced in the 6 monkeys subcutaneously vaccinated with killed arthrospores. Four of the 6 monkeys in each of the other immunized groups also survived. Only 2 of the 6 challenged controls survived. All animals were infected as a result of challenge. No dissemination was observed in animals immunized with the boivin-type fraction. The skin test reaction may be useful in detecting a response to immunization, but thus far the agar gel precipitin and CF reactions have not proved useful in detecting resistance to infection.