The authors attempted to study which parts of the lipid molecules are most responsible for an increase in the sensitivity of the tumor cell to the cytostatic and membrano-toxic action of natural spleen effectors. Use was made of different combinations of egg phosphatidylcholine, phosphatidylethanolamine, cardiolipin, dipalmitoyl phosphatidylcholine and a mixture of bovine brain gangliosides. Introduction of phosphatidylethanolamine into the membrane of tumor cells increased, that of phosphatidylcholine and cardiolipin did not change whereas introduction of a mixture of brain gangliosides reduced their sensitivity to spleen effectors. Introduction of brain gangliosides together with egg phosphatidylcholine into the membrane of the target cell increased whereas that together with dipalmitoyl phosphatidylcholine reduced its sensitivity to spleen effectors. It is concluded that the increase in the sensitivity of the tumor cell primarily depends on changes in the lipid template of its membrane, induced by unsaturated fatty acids of egg phosphatidylcholine, and to a less degree on the properties of carbohydrate heads of gangliosides.