Addition of diluted blood serum from tumor-bearing rats stimulated significantly the synthesis of cholesteryl esters from labeled cholesterol and endogenous fatty acids in the cytosol derived from normal rat liver. With both Zajdela and Walker transplantable tumors this effect was found to be associated with the most intensive period of tumor growth. During chemical carcinogenesis induced by a single subcutaneous administration of benzo(a)pyrene the stimulating effect of sera was found to precede several weeks the appearance of palpable tumors and persisted during the period of progressive tumor growth. With all tumors used, sera in ultimate stages of tumor growth failed to show a stimulating effect. The stimulating effect was due to the presence of a yet unidentified lipid. Higher quantities of this substance may appear in the serum of tumor-bearing animals to meet higher requirements for cholesteryl esters during tumor growth. The stimulating effect of the blood serum on cholesteryl esters may be a useful marker of malignant tumors in humans.