The author has studied on the Capillary Permeability Promoting (C. P. P.) Action of cancer toxin and its genesis, obtaining following results. 1) Cancer extract, toxohormone and urine extract of cancer patients have C. P. P. action. 2) A capillary permeable substance, which is heat-resisting and diffusive, is extracted from cancer tissue. This substance has a notable difference in sampling as compared with the capillary permeability of toxohormone and urine extract of cancer patients, and is contained in the globulin fraction on ammonium sulfate method. The substance is found to belong in the polypeptides group according to the results obtained above and to proteineand amino acid tests. 3) In the serum of rabbit administered cancer toxin C. P. P. action and fibrinolysis are observed, and these two actions are significantly interrelated in sampling 4) The C. P. P. action and fibrinolysis produced by administration of cancer toxin are thought to be derived from activation of profibrinolysin to fibrinolysin followed by appearance of permeability of cappillaries. 5) On the study of C. P. P. substance in cancer tissue, used radioactive isotope P(32) as a tracer, function of reticuloendothelial cells is declined and that of mesenchymal cells is accelerated. From the expermental results described above, the action of C. P. P. substance in cancer tissue is considered to play an important role on the growth of cancer and occurrence of cachexia, on thinking the fibrinolysis and the functional decline of cells of the reticuloendothelial system.