Earlier as well as recent evidence obtained in our laboratory is reviewed indicating that specific changes in cholesteryl ester metabolism are related to tumor growth. Some properties of cholesteryl 14-methylhexadecanoate, an unknown lipid discovered and identified in our laboratory are described and its fundamental role as an essential cofactor in protein synthesis is emphasized. Increased quantities of this ester are present in tissues of tumor-bearing animals and the elevation of its level in serum is proportional to the tumor growth. Evidence has been presented that it may be a useful and reliable marker of malignant tumors in humans. This compound is synthesized in liver cells by enzymes that were recently purified to homogeneity from rat liver. The activity of these enzymes is significantly enhanced in tumor-bearing rats in a close proportion to the tumor growth. Activity of these preparations from normal rat liver may be greatly enhanced by the addition in vitro of serum from cancer patients. Since the enzymes are of lecithin-cholesterol acyl transferase type, apparently cancer serum contains some peculiar molecular species of these phospholipids. Because these lipids may be specific products of tumor cells, attempts are now in progress in this laboratory to identify them and the possibility is studied for their utilization as specific tumor markers.

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