Heinz bodies produced in vitro by X-irradiation or hydroxylamine hydrochloride were separated and collected. The lipids extracted from these Heinz bodies were analyzed as to their lipid and fatty acid composition by thin layer chromatography and gas-liquid chromatography. The purpose of the present experiment is to compare the composition of lipids and fatty acids in the Heinz bodies produced by the two different methods. In addition, the membranes of the untreated red cells were similarly treated and analyzed for comparison. 1. It was found that there were differences in the composition of lipids and fatty acids derived from Heinz bodies under different circumstances in which they were formed, even though they might appear morphologically identical on stained preparations. Namely, Heinz bodies contained a lipid fraction whose chemical composition was not necessarily the same and it was suggested that they were the degenerative products having the same characteristics. 2. Analysis of lipids of Heinz bodies indicated a marked decrease of the phosphatidyl ethanolamine ratio in comparison with the lipid fraction of the normal red cell membranes. 3. With respect to the fatty acid composition of Heinz bodies, there were a decrease of the stearic acid ratio and an increase of the oleic and linoleic acid ratios, resulting in a marked increase of the total unsaturated fatty acid ratio. 4. The development of X-irradiation anemia is due not only to an impaired erythropoiesis in the hematopoietic tissue but to an accelerated destruction of red cells in the peripheral blood. It is postulated that the peripheral red cells are rendered fragile by X-irradiation through an alteration of the lipid composition ratio of the cell membrane.