Fatty acids extracted from Heinz bodies were aided to Ehrlich ascites tumor cells and HeLa cells in tissue culture at various final concentrations of 0.2, 0.1, 0.05, 0.025%, and their effects on these cultured cells were morphologically observed. It was shown that fatty acids derived from Heinz bodies exerted as nearly a potent cytotoxicity as OX substance oa the cultured tumor cells. At the concentration of 0.2 and 0.1%, immediately following thier addition, almost all the cells showed marked degenerative changes such as bubbling and blister formation, lipid granulation in the cytoplasm and pyknosis of the nuclei and this was soon followed by cell floating and death. And even at the lower concentration of 0.05 and 0.025% the cell degeneration was observed in 3 hours and 6 hours respectively and became floating within 24 hours. From these results, it is considered that the formation of Heinz bodies may be a biological reaction as the body defense mechanism,