1- Acylglycerophosphorylcholine acyltransferase in platelets and liver microsomes showed a selectivity for polyunsaturated fatty acids. The acyl-donor specificity of the enzyme for 1- acylglycerophosphorylcholine and also 1- acylglycerophosphorylethanolamine was high for n-3 eicosapentaenoyl -CoA and docosahexaenoyl -CoA, and highest for arachidonyl-CoA. Further, the enzyme activity was higher for n-6 eicosatrienoyl -CoA than for n-3 and n-9 eicosatrienoyl -CoAs. It appears that the acyltransferase has a predominant selectivity for n-6 series, but the selectivity is not limited to this series. Acyl-CoA synthetase of liver microsomes and platelets also exhibited a high selectivity for n-3 polyunsaturated fatty acids as well as n-6. In platelets, the acyl-CoA synthetase activity preferential to polyunsaturated fatty acids (n-3 and n-6) was found by competitive aspects of incorporation, in addition to the activity preferential to palmitic acid. In any case, it is suggested that n-3 polyunsaturated fatty acids are easily incorporated into platelet and liver phospholipids.