Lipid was extracted from lyophilized samples of cow, goat, sheep, horse, and human milk. The cephalins were isolated by thin-layer chromatography and ascertained to be the major, but variable component of all milk phospholipids examined except in the case of the horse. The predominant cephalin type was found to be phosphatidyl ethanolamine (PE), while phosphatidyl serine (PS) was determined to be either absent or to account for approximately 20% of the total cephalins. Lysocephalins were generally absent. The fatty acid composition of PE and PS was characterized by high levels of oleic and palmitic acids and by the virtual lack of long-chain polyunsaturated fatty acids. Individual differences in fatty acid content were slight among members of the same species, but human milk PE showed considerable variations in composition. PE was determined to be more highly unsaturated than PS with the unsaturated fatty acids occurring primarily at the β-position of the PE molecule. The results obtained on the chemical structure of milk cephalins have been discussed in relationship to the suggested activity of these compounds on the blood coagulation process in vitro.