An immunization trial carried out on 1310 persons professionally exposed to Q fever confirmed the suitability of a chemovaccine for field use. Its immunogenicity and reactogenicity differed in three groups of subjects who differed in their previous experience of Q fever and who were given different lots of the vaccine. There was some correlation between reactogenicity of the vaccine and pre-immunization positivity in the microagglutination and skin tests as indicators of a previous exposure. The skin test was more reliable for predicting post-immunization reactions.