The influence of both polar head and acyl chain of lysophospholipid on the activity of partially purified acyl-CoA:lysolecithin acyltransferase from rabbit lung was studied. It was concluded that the presence of methyl groups on the nitrogen of the base was essential for recognition of lysophospholipid as substrate by the enzyme. With respect to the acyl chain length and saturation, the activity followed the order: 16:0 approximately equal to 18:1 greater than 14:0 greater than greater than greater than 18:0 approximately equal to 12:0. Also, the effect on the activity of the acyl chain on acyl-CoA was studied. The activity showed great selectivity for saturated acyl-CoAs. The activity with polyunsaturated fatty acids was very low and in the case of arachidonoyl-CoA was almost negligible. The comparison between crude microsomal preparations and partially purified preparations allowed to suggest that it could exist two different acyl-CoA:lysolecithin acyltransferases differing in their selectivity towards saturated and unsaturated fatty acids.