84 PA THOGENIC BA CFERIA. A new principle discovered by Pfeiffer, and bearing directly upon the theories of immunity, is that the se- rum of animals immunized to certain diseases (cholera and typhoid) contains a germicidal substance. Metchni- koff has tried to show that the action of this body depends upon solution of the leucocytes, but Pfeiffer has disproved this by showing that the liquor puris from abscesses oc- curring in the experiment-animals did not contain the active substance.1 The work of van de Velde2 is very interesting. An animal immunized by progressively increasing doses of strong filtered toxin produced a serum possessed of pow- erful anti-infectious and antitoxic powers; one immu- nized by the introduction into its body of the washed, precipitated bodies of diphtheria bacilli collected by ni- tration furnished a serum of appreciable anti-infectious, but no antitoxic properties; one immunized by the use of bacillus cultures developed antitoxic and anti-infectious serum identical with the first described; one immunized to weak toxin furnished serum of considerable anti-infec- tious, but slight antitoxic power, and still another ani- mal that received toxin that had been heated developed neither anti-infectious nor antitoxic serum. Seeing that the serums commercially manufactured are made by the use of strong filtered toxin, van de Velde examined a number of samples purchased in the market, and found that they were all possessed of both antitoxic and anti-infectious properties. It is important to remem- ber the presence of both of these properties in the serum, as the successful use of the agent for immunizing depends upon the presence of the one, and the use in treatment upon the presence of the other. Immunity and antitoxins stand in unknown relation- ship to one another. That an animal has considerable antitoxin in its blood is no guarantee that it is immune. I have seen a horse in each c.cm. of whose blood there 1 Centralbl. f. Bakt. u. Parasitenk., Bd. xix., Nos. 14 and 15. 2 Ibid., Nov. 24, 1897, Bd. xxii., Nos. 18 and 19.