Abstract 1. (1) Long-chain carnitine acyl transferase activity in red cell membranes was demonstrated by their capacity to incorporate [1- 14C]oleic acid and [1- 14C]linoleic acid into the respective acylcarnitines. The addition of (−)-carnitine to red cell membranes enhanced [1- 14C]palmitate incorporation into palmitylcarnitine, whereas (+)-carnitine was without effect. (+)-Palmitylcarnitine resulted in a depression of [1- 14C]palmitate incorporation into palmitylcarnitine, and this inhibition was reversed by the addition of (−)-carnitine. 2. (2) Fatty acylcarnitine derivatives were shown not to be obligatory intermediates in the incorporation of fatty acyi CoA into lecithin, and the addition of (−)-carnitine increased incorporation of [1- 14C]palmitate into palmitylcarnitine while decreasing incorporation of [1- 14C]palmitate into lecithin. 3. (3) ‘Methyltransferase’ activity was demonstrated in human and sheep erythrocytes. Significant incorporation into lecithin, sphingomyelin, methylated phosphatidylethanolamine derivatives and lysolecithin was found with S-[ Me- 14C]adenosylmethionine. Methionine-activating enzyme activity was absent.