SummaryMuch work has been done about the function of the spleen, and hitherto our really positive knowledge has remained still incomplete. The different functions so far known are discussed.Detailed study of the influence of regular bloodtapping and of continued hæmolysis on the spleen. After a certain time of bloodtapping the spleen becomes smaller and becomes finally so small that the weight is reduced to less than 50 mgr. These spleens are so atrophic that it can scarcely be imagined that the normal hæmolysis of blood can be located in this atrophic organ. The various depotfunctions of the spleen have become superfluous. Of the other functions described are two dealing with bloodformation. The first is that the spleen should exercise a stimulating influence on the bloodformation in the bone marrow. A second conception presumes that the spleen should exercise a checking influence on the outgoings of the erythrocytes from their places of formation in the restituting blood. The spleen would prevent the bone marrow from functioning too actively. The first conception is hard to bring into accord with the almost entire disappearance of the spleen. For it is the very bone marrow which must be whipped up to maximal activity, and consequently an extra function of the spleen might be expected, On the other hand it is logical that with top-anemia the checking function will fall out. Here all bloodcells made, must be brought as quickly as possible in the blood channel.In a second group of experiments was shown that in anemic rabbits injection of splenic tissue recovering from the existing anemia is slower than normal.At the same time We see a reticulocytosis not as a temporary phenomenon, but persists in proportion to the existing anemia.Finally a new untill now unknown function of the spleen is discussed. In the bone marrow we can find anticomplementary factors which are regulated by the spleen.