The author studied the trends of serum cholesterol, lipoprotein, and proteins, mainly in leukemia and various blood diseases, and obtained the following results. The subjects of the present investigation were the cases diagnosed as such by bonemarrow tissue cultures and hematological examinations. 1. The cholesterol value in leukemia is almost at the normal level, showing not any marked difference by the type of the disease, but the ratio of β- and α- lipoproteins generally presents an increasing tendency. As for the trend of serum proteins on the whole a decrease in total concentration and albumin and an increase in γ-globulin are the common changes, and the differences by the type can not be considered to be marked. Only in mono cytic leukemia the decrease in albumin and the increase in γ-globulin are marked. 2. Hypoplastic anemia shows the normal level of cholesterol, but it is interesting to note that the total cholesterol, ester cholesterol as well as the ratio of ester to total chole sterol are all in the decreasing order of type I>type III>type IV. Moreover, a decrease in α-globulin and an increase in γ-globulin in the serum protein content can be observed. As for the other anemias Banti's disease shows an increase in the lipoprotein ratio, a marked decrease in serum protein, and an increase in γ-globulin; idiopathic hypochromic anemia a decrease in serum proteins; a single case of hemolytic anemia an increase in cholesterol, a decrease in the serum protein content and albumin fraction and an increase in γ-globulin. In addition, in one case of hookworm anemia an extremely high value of the lipoprotein ratio, a decrease in albumin and an increase in γ-globulin can be recognized. 3. In cancer a decrease in cholesterol, serum protein content, and albumin fraction and an increase in γ-globulin can be recognized, but as for the differences in the site only hapatoma shows a marked change in serum protein and fractions. 4. Choresterol in idiopathic thrombocytopenic purpura is decreased, and in one of them a decrease in albumin of serum protein fractions and a slight increase in γ-and β- globulin can be observed. In lymphogranulomatosis a decrease in albumin and an increase in γ-globulin are marked, and in one case of reticulum cell sarcoma a decrease in cholesterol, the serum protein content, and albumin as well as an increase in γ-globulin can be recognized.