THE ANTICOMPLEMENTARY reaction is the inhibition of hemolysis in the antigen-free mixture of serum and complement used as control in complement-fixation tests. Such a serum is not suited for a Wassermann or a Kolmer test. In dealing with anticomplementary reactions, one must differentiate between extrinsic and intrinsic anticomplementary reactions. 1. The "extrinsic" (or "exogenous" or "artificial") anticomplementary reaction is usually caused by bacterial or chemical contamination. Use of old sera and impurities, e.g., acids, alkalis, anticoagulants (oxalates, citrates, heparin), arsenicals, tannin, alcohol, etc., may provoke anticomplementary reaction in any serum.1 All these extrinsic anticomplementary reactions can be avoided by proper laboratory technique. 2. The "intrinsic" (or "endogenous" or "autonomous" or "natural") anticomplementary reaction is not caused by faulty laboratory technique and cannot be avoided by correct laboratory technique but is an intrinsic quality of the serum itself. A test for differentiating these two groups is the