Summary and Conclusions 1. Active cases of coccidioidal granuloma, prior to the final fatal stages give a positive skin reaction. 2. Tuberculosis patients tested with coccidioidin as a rule give negative reactions. From most of the positive reactions observed in this study it has been possible to obtain a history compatible with exposure to coccidioides infection. 3. Guinea pigs experimentally inoculated with Mycobacterium tuberculosis give a negative coccidioidin skin test and a positive tuberculin test. 4. Guinea pigs experimentally inoculated with Coccidioides immitis give a positive coccidioidin skin test and react negatively to tuberculin skin tests. These results indicate that the coccidioidin skin test possesses a high degree of specificity and parallels in many respects the tuberculin test in its mode of action. It can be used as an aid to diagnosis but probably no more reliance should be placed on it than is usually given the tuberculin test.