The reasons for the failure of such sera are not entirely clear. The following are some that have been offered: (1) Amboceptors do not appear to be present in very large amount so that relatively large injections of blood are necessary, which is not without risk in itself. (2) Since the complement is furnished by the blood of the animal to be treated, there may not be enough of this to unite with a sufficient quantity of amboceptor to destroy all the bacteria present, hence the disease is continued by those that escape. (3) Or the complement may not be of the right kind to unite with the amboceptor introduced, since this is derived from the blood of a _heterologous_ (“other kind”) species. In hog-cholera serum, if it is bactericidal, this difficulty is removed by using blood of a _homologous_ (“same kind”) animal. Hence Ehrlich suggested the use of apes for preparing bactericidal sera for human beings. The good results which have been reported in the treatment of human beings with the serum of persons convalescing from the same disease indicate that this lack of proper complement for the given amboceptor is probably a chief factor in the failure of sera from lower animals. (4) The bacteria may be localized in tissues (lymph glands), within cavities (cranial, peritoneal), in hollow organs (alimentary tract), etc., so that it is not possible to get at them with sufficient serum to destroy all. This difficulty is obviated by injecting directly into the spinal canal when Flexner’s antimeningococcus serum is used. (5) Even if the bacteria are dissolved it does not necessarily follow that their _endotoxins_ are destroyed. These may be merely liberated and add to the danger of the patient, though this does not appear to be a valid objection. (6) Complement which is present in such a large excess of amboceptor may just as well unite with amboceptor which is not united to the bacteria to be destroyed as with that which is, and hence be actually prevented from dissolving the bacteria. Therefore it is difficult to standardize the serum to get a proper amount of amboceptor for the complement present. COMPLEMENT-FIXATION TEST. Although little practical use has been made of bactericidal sera, the discovery of receptors of this class and the peculiar relations between the antigen, amboceptor and complement have resulted in developing one of the most delicate and accurate methods for the diagnosis of disease and for the recognition of small amounts of specific protein that is in use today.