The evidence presently available indicates that the final step in the synthesis of triglycerides in liver is the esterification of an Q:,/3-diglyceride by an activated fatty acid (I, 2). Weiss, Kennedy, and Kiyasu (1, 3) have demonstrated this esterification reaction in a particulate preparation of chicken liver with palmityl coenzyme A and various diglycerides as substrates. Recently, Steinberg, Vaughan, and Margolis (4) have presented evidence consistent with a precursor role for diglyceride in triglyceride synthesis by homogenates of rat epididymal fat pads. An analysis of the triglycerides of animal fat has revealed a relative preponderance of unsaturated fatty acids in the/3 position of the glyceride in all animals except the pig (5-7). This composition might be governed by the specificity of the enzyme (s) catalyzing the final step in their synthesis. In chicken fat, palmitic acid and oleic acid together comprise more than half the total fatty acids of the triglycerides and are the most common saturated and unsaturated fatty acids.’In this paper the specificity of esterification of diglycerides is examined in a particulate enzyme system obtained from chicken fat. The diglycerides and activated fatty acids used in these studies contained oleic and palmitic acid.