A study was carried out in order to investigate whether the abnormalin vitroturnover of fatty acids in the phospholipids of the red blood cell membranes of cystic fibrosis patients is intrinsic to the membrane, or whether it is induced by extrinsic serum factors. Red blood cells of cystic fibrosis patients and healthy subjects were labeledin vitrowith [14C]linoleic acid, bound to albumin. The labeled cells were reincubated in autologous and homologous serum. The radioactivity present in the serum lipids and in the major phospholipid fractions of the red cell membranes was measured.Conclusions of this study are: 1) not all of the cystic fibrosis patients examined individually show an abnormalin vitroturnover of the red cell fatty acids, although they all presented abnormal fatty acid patterns for the red blood cell phospholipids, ±