The development of new analytic and preparative techniques in the field of protein chemistry has essentially extended our knowledge about the variety of human plasma proteins in the last 15 years. In many cases plasma proteins have been particularly determined by immunologic techniques, partially highly purified and physicochemically well characterized, before knowing the biological function. To these proteins belong among others the alpha 1-antitrypsin, Cl-inactivator, alpha 2-macroglobulin, Gc-globulin and the cold-insoluble globulin. Today we know more than 100 proteins being isolated from human plasma and among these are nearly 20, of which the biological function is not yet known. To this group are belonging proteins, which are known since many years, like the alpha 1-acid glycoprotein and the C-reactive proteins as well as proteins, which have only been described in the last years and which partially have an interesting chemical structure, e.g. the histidinerich 3,8S-alpha 2-glycoprotein and the leucine-rich 3,1S-alpha 2-glycoprotein, of which every fifth amino acid is formed by leucine. It is to be hoped that the special chemical structure of some of these human plasma proteins as well as the quantitative immunologic determination in different patient sera will give hints to their biological function.