One phenomenon not fully explained at the present is that mammalian tissue phospholipids contain a much higher content of long-chain polyunsaturated acids at the 2-position than do the triacylglycerols% 4 In fact, MATTSON AND VOLPENHEIN~ found in plant seed oils that acids with a chain length of more than 18 carbon atoms, regardless of their ethylenic bond content, were esterified almost exclusively at the primary position (s). One explanation would be that Reaction 4 selects only those diacylglycerols that do not contain polyunsaturated acids in accord with an earlier suggestion2. Alternatively, Reactions I and 2 may not effectively use these acids and thus produce diacylglycerols containing predominantly palmitate, oleate and linoleate. Polyunsaturated acids could then enter phosphatides via acyltransferasecatalyzed reactions (Reaction 6) with the monoacyl derivative+‘. The following experiments were designed to test the selectivity of the transfer of these acids in Reaction z in comparison to that for Reaction 6. I-Acyl-GP was prepared by phosphorylation of monooleoylglycerol with POCl, in chloroforms and was isolated as the ethanol-insoluble sodium salt (phosphorus: ester= I. 2). Acyl-CoA derivatives were prepared by condensation of the acid chloride with CoA as described by REITZ et a1. 9. Acid chlorides were obtained commercially or by treating the free acids (Hormel Institute) with oxalyl chloride. Quantitative gas chromatography of the methyl esters prepared from acyl-CoA’s indicated purities of greater than 950/b for the 20: 3, 20: 4, 20: 5 and 22: I and greater than 90 ($; for the 22: 6 thiol esters. All other CoA thiol esters were more than 999& pure. I-Acyl-GPC was prepared by treatment of egg phosphatidylcholine with phospholipase A (ref. IO) and was isolated by column chromatography on silicic acid. Rat and guinea-pig liver microsomes were prepared as described earlier6 and stored at-15’in 0.25 M sucrose-I mM EDTA (25-40 mg protein/ml). Rat liver microsomes were treated with 0.1 vol. IO mM DFP in propylene glycol for 30 min at room temperature prior to the assay for acyltransferase activity. Acyl-transfer reactions were assayed by following the increase in absorbance at 412 my produced by the reaction of the released mer-Abbreviations: GP, L-glycerol-j-phosphate; GPC, L-glycerol-3-phosphorylcholine; DTNB, 5, 5’-dithiobis (z-nitrobenzoic acid).