Aerosol and subcutaneous vaccination with killed arthrosporess and subcutaneous vaccination using a boivin-type fraction were compared for their efficacy in protecting rhesus monkeys against lethal aerosol challenge with C. immitis. Complete protection against lethal challenge was produced in the six monkeys subcutaneously vaccinated with killed arthrospores. Four of the six monkeys in each of the other immunized groups also survived. Only two of the six 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.