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colony typical hooked Leptospira icterohaemorrhagiae was virulent for hamsters and guinea pigs. On cultivation, it was gradually replaced by a serologically identical small-colony avirulent straight mutant. The hooked virulent form was selected in vivo.

HUDDLESON et al. (3), in 1932, described an ,,Qpsono-cytophagicreaction" which they claimed to be suitable for the laboratory-diagnostic of brucellosis. It was based on the experience that addition, of sodium-citrate in a definite amount to the whole-blood, eliminates a great deal of non-specific phagocyto~is, whereas specific opsonic action remains unchanged.This test has only been applied in practice in U.S.A. Unfortunately several investigators were unable to confirm the reliability of HUDD-LESON'S reaction, especially with regard to the specificity of its results, nor did I. acquire data, which were satisfactory enough to recommend the test for routine practice.Studying a large material I was able to establish the specificity of phagocytosis in the b.loodserum of men or animals, suffering from bruceUa-infection, after the sera had lost their complement by heating.Here upon I have based a serum test, which I called ,,tropin-reaction" the description of which first has been published in 1940 (2). In 1941 JEgSILD (4) published a similar method, depending upon the same principle, but applying a different technique. His results in brucellosis of man are claimed to be very satisfactory.The technique of the tropin-reaction, sucli as it is used by me, is the following:S e r a to be tested are inactivated by heating during 30 minutes at 56 ° C.The phagocyting 1 e u c o c y t e s may originate either from man or ,from animals~ Five ccm blood collected in glass vials conta~ing 0.2 ccm of a 20 percent solution of sodium-citrate (in physiological salt solution) are centrifugated and the sediment is washed once with

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