The revival of an ancient principle of treatment practised by Mithridates in his empire is of recent occurrence. We read that "Mithridates believed it possible to render himself proof against all forms of poison by the constant and daily use of small doses of various poisonous principles, and he compounded a general antidote by combining all forms of poison then known. He mixed with this the blood of geese and used the remedy for snake bite, reasoning that the geese must have some antidote for snake venom in their blood, because one of the chief articles of their diet was the viper, to whose bite they were very resistant." At the present time we use the serum from the blood of animals that have been treated with a killed culture of bacteria, or the toxins caused by bacteria, until the animals become immune to the kind of bacteria or toxins used. Ehrlich explains this immunity by his side-chain theory. His idea is that the body cells have groups of characteristics which may link them to certain substances. Toxins contain two groups of properties, the toxaphore group, which exerts its influence on the body cells, and the haptophore group, which combines the toxaphore group or toxic element with the haptophore group in the tissue cells, forming a side chain to the cells. The haptophore groups of the cells are destroyed, but nature quickly replaces them, producing more than were destroyed, the excess being liberated into the blood. This excess is called antitoxin. The antitoxin has an affinity for the haptophore group in the toxin, and by combining with the latter in the circulation protects the tissue cells by rendering the toxin inert. The injection of antitoxin gives immunity - as in tetanus - if administered before the toxin is introduced into the system, or cures - as in diphtheria, - in some cases, if administered after the toxin has entered the system. This is antitoxic immunity. Bactericidal immunity is more complex. When bacteria enter the body they cause the tissue cells and other cells to throw off an immune body (amboceptor) which can destroy the bacteria only by linking them to the co?nplement which is contained in the bodies of all beings. The immune body is the protective agent. Much could be hoped from bactericidal sera could they be obtained so readily as antitoxin, but great difficulty has been experienced in procuring such sera. We have then two kinds of sera: antitoxic, such as antitetanus, antidiphtheritic, and anticellular; and antibacterial, such as antipneumo-coccic, antityphoid, and antiplague.