The author studied the toxic substance in the serum of patients with malignant tumors frequently encountered in the field of internal medicine such as gastric cancer as a principal one and liver cancer, cancer of large intestine, and lymphosarcoma by observing the tissue growth and neutrophil function in rabbit bone-massow tissue cnlture and also the erythrocyte series by culture in the fluid medium. The following are the results. 1. The tissue growth in the case of the bone-marrow tissue culture loaded with the serum of cancer patients is diminished as compared with that of the control. 2. The wandering velocity of pseudeoesinophils in the majority of the cases loaded with the serum of cancer patients is lower than that of the control. 3. As for the vital staining of pseudoeosinophils in the cases loaded with cancer patient's serum, the cells are stained deeper at an earlier stage as compared with the control, proving the diminution of the cell functions. 4. In the fluid medium culture loaded with cancer patient's serum erythrocytes generally tend to increase in number to a greater extent than in the control, but the Hb content on the whole is diminished. 5. From these findings it is assumed that the toxic substance contained in cancerous sera brings about the diminution of the leucocyte functions by its direction on the bone marrow while the erythrocyte series besides its direct action on the bone marrow there is other principal factor.