When we recent,ly investigated the blood-clotting activity of individual and combined phospholipids
(1)) PE' and PS in naturally occurring phospholipid mixtures had to be determined. These mixtures only showed the high activity of the "natural lipid activator" from platelets if they contained at least small amounts of PS. In soybean cephalin, however, which strongly promotes blood clotting, PS could often not be detected (2). Thus the necessity arose to elaborate a method for the exact determination of smallest amounts of PS and also PE. The various methods described in the literature proved inadequate for this purpose. The usual approach is to hydrolyze the phospholipid sample, to separate ethanolamine and serine, and to determine their quantities. In his excellent survey of the abundant earlier literature
McKibbin
(3) evaluated one method as "sufficiently satisfactory." Dittmer et nl. ( 4) stated that, '(an improved method of hydrolysis . . . is obviously needed." Since then a new technique has been suggested by Magee et al. ( 5) and the remarkable method of Dawson (6), which even permits separate identification of the ethanolamine and serine originating from the diester and the plasmalogen part of the phospholipid mixtures, was published. In the present paper we describe a simple and rapid technique for the determination of ethanolamine and serine from 5-to 15-mg samples of phospholipid. This method gives consistent results when tested on synthetic and native phospholipids of animal and vegetable origin.