Percentage of fatty acids in phospholipids of red cell membranes from cord blood of 13 full-term and 11 premature infants is studied. Percentage of 22:5n6, final metabolite of n6 series, was significantly diminished in premature infants, whereas the percentage of arachidonic acid was significantly increased in the same group. The possibility that an enzymatic abnormality be present in desaturation from 22:4n6 to 22:5n6 is speculated, as well as of its physiologic implications.