A "double-blind" study of the Kveim test on 86 patients with active and with inactive sarcoidosis on and off steroid therapy, and on controls including those with lymphomas, normal subjects with negative tuberculin reactions, and patients with pulmonary tuberculosis, led to the following conclusions: When properly performed the Kveim test is a specific and reliable diagnostic tool for detecting cases of active sarcoidosis. Patients with inactive sarcoidosis may or may not have positive Kveim reactions after the injection of a good sarcoid antigen. When positive, the reaction may be atypical. Nonsarcoid patients, including those with lymphomas, negative tuberculin reactions, and with tuberculosis, do not have positive Kveim reactions when injected with a potent sarcoid antigen. The antigen should be prepared from sarcoid tissue and stored under deep-freeze refrigeration. Antigen prepared from normal tissue cannot be used as diagnostic antigen, but may be used in conjunction with an antigen prepared from sarcoid tissue. Kveim antigen, pretested for the absence of contaminating foreign material, may produce in patients with active sarcoidosis reactions resembling "foreign-body giant cell granuloma." Such reactions may be related to special hypersensitivity and, perhaps, to the activity of the disease.