Since blood platelet phosphatides exhibit specialized activity in the generation of plasma thromboplastin, their fatty acid composition was studied by means of gas-liquid chromatography. The phospholipids were separated on a silicic acid column and the fractions methylated. PE-PS fractions (active in clotting) contained primarily arachidonic, stearic and oleic acids. Lecithin-sphingomyelin eluates (inactive in clotting) contained principally palmitic, stearic, and oleic acids. Most notable was the association of arachidonic acid with fractions possessing maximals thromboplastic activity. These findings support the concept that the degree of unsaturation of the fatty acid on the phospholipid molecule is directly correlated with the promotion of blood thromboplastin.