The phospholipid class composition and the ratio of the amounts of two sphingomyelin fractions, supposed to have different fatty acid compositions, were determined by quantitative thin-layer chromatography on the erythrocytes and plasma of four representative mammalian species, man, pig, cow and rabbit. A remarkable species difference was confirmed on the phospholipid class compositions between bovine erythrocyte and those of three other species. The phospholipid distribution in plasma was rather relatively similar each other in these species, with the predominant presence of choline phospholipids. The ratio of two sphingomyelin fractions varied appreciably from species to species. The bovine erythrocyte gave the highest value whereas the bovine plasma gave the lowest value of the F/S fractional ratio among these four species.