Insulin binding was studied using erythrocytes obtained from two homozygotes of familial partial deficiency of plasma lecithin: cholesterol acyltransferase (LCAT). Both patients revealed corneal opacity, plasma LCAT activity with 14-15% of the normal value, target cells and abnormalities of plasma lipoprotein composition with a reduced ester ratio of cholesterol. In addition, abnormalities of lipid composition of red cell ghosts were observed: cells obtained from both patient showed increased amounts of cholesterol and lecithin and an increase in the lecithin/sphingomyelin ratio. Insulin binding to the red cells was also increased, mainly due to an increase in binding affinity without a significant change in receptor number. Further studies are needed to confirm whether the increase in binding affinity is due to the change in lipid composition of the red cell membrane.