which is capable also of uniting with complement. The indicator is put into the first mixture and the whole is again incubated and examined. If the mixture is _clear_ and _colorless_ with a _deposit of red corpuscles_ at the bottom, that would mean that the complement had been bound to the first complex, since it was not free to unite with the second sheep’s corpuscles (antigen)--rabbit serum (hemolytic amboceptor) complex--and destroy the corpuscles. Hence if the complement is bound in the first instance, the _specific amboceptor_ for the first antigen must have been present in the blood, that is, the animal was infected with the organism in question. Such a reaction is called a “positive” test. On the other hand, if the final solution is _clear_ but of a _red_ color, that would mean that complement must have united with the corpuscles--hemolytic amboceptor system--and destroyed the corpuscles in order to cause the _clear red_ solution of hemoglobin. If complement united with this system it could not have united with the first system, hence there was no _specific amboceptor_ there to bind it; no specific amboceptor in the animal’s blood, means no infection. Hence a _red solution_ is a “negative test.” The scheme for the test may be outlined as follows: Antigen + Patient’s Serum, heated + Complement (specific for (unknown amboceptor) (derived from the amboceptor guinea pig’s serum) sought) Incubate one-half hour in a water bath or one hour in an incubator. Then add the indicator which is Antigen + Amboceptor (red blood corpuscles) (for corpuscles, serum of a rabbit immunized against the red corpuscles) Incubate as above. In practice all the different ingredients must be accurately tested, standardized and used in exact quantities, and tests must also be run as controls with a known normal blood of an animal of the same species as the one examined and with a known positive blood. It should be stated that in one variety of the Complement-Fixation Test, namely, the “Wassermann Test for Syphilis” in human beings, an antigen is used which is not derived from the specific organism (_Treponema pallidum_) which causes the disease nor even from syphilitic tissue. It has been determined that alcohol will extract from certain tissues, _human or animal_, substances which _act specifically_ in combining with the syphilitic amboceptor present in the blood. Alcoholic extracts of beef heart are most commonly used. Details of this test may be learned in the advanced course in Immunity and Serum Therapy.