The phospholipid composition of hepatic microsomal fractions from different developmental stages of embryonic chick was established. The major components were phosphatidylcholine (approx. 66%), phosphatidylethanolamine plus phosphatidylserine (approx. 21%) and sphingomyelin (approx. 9%). 2. There were no significant changes in the phospholipid composition during embryonic development from 9 to 20 days. 3. When microsomal subfractions were prepared it was found that the smooth-microsomal fractions (Ia and Ib) had a significantly greater sphingomyelin content than the rough-microsomal fraction (II). This was compensated by a lower phosphatidylcholine content in fractions Ia and Ib and an increase of phosphatidylcholine in fraction II. 4. The significance of the differences in the phospholipid composition of smooth and rough microsomes is discussed with particular reference to the origin and interrelation of smooth and rough endoplasmic reticulum.
