1. A fresh emulsion of tubercle bacilli in salt solution when used as the antigen will give about 90 % of positive results in cases of pulmonary tuberculosis, whilst in healthy persons the result is negative.2. In cases suspected of pulmonary tuberculosis, the results of this test conform closely to those given by a combination of the opsonic index test with injections of tuberculin, and so may be of value in diagnosis.3. Diseases other than tuberculosis have always been negative, but only a small number of such cases have been examined.4. An emulsion of tubercle bacilli when used as an antigen must be freshly prepared immediately before use, and should be made from a young subculture.5. Owing to the differences in complements, they should always be titrated in the presence of the antigen, and probably also in the presence of both antigen and normal serum.6. Exercise does not seem to affect the result of the reaction in any way, but a course of tuberculin treatment leads to an increase in the complement-fixing bodies of the serum in some cases.