The following phosphatides (in approximate order of concentration) were studied in cells of the Ehrlich ascites carcinoma incubated in a medium containing inorganic P32: lecithin > sphingomyelin > phosphatidyl ethanolamine = phosphatidyl inositol = phosphatidic acid > choline plasmalogen = phosphatidyl serine > ethanolamine plasmalogen. The specific radioactivity of the diacyl-glycerophosphatide fraction exceeded that of both the plasmalogen and the sphingomyelin – glycerol ether phosphatide fraction, the specific radioactivity of the individual phosphatides being as follows: phosphatidic acid > phosphatidyl inositol > ethanolamine plasmalogen > phosphatidyl ethanolamine = choline plasmalogen = lecithin > sphingomyelin. The microsomal fraction contained more phospholipid, followed by the mitochondrial and nuclear fractions, in that order. The specific radioactivities of the phospholipids of the microsomes and nuclei were greater than that of the mitochondria, chiefly because of the high specific radioactivity of the diacylglycerophosphatide fraction. The high specific radioactivity of the diacylglycerophosphatides was largely the result of a very active incorporation of inorganic P32into phosphatidic acid, particularly in the microsomal fraction. The significance of these findings is discussed.