The methylation of erythrocyte membrane proteins has been investigated with fractionated reversible and irreversible sickle erythrocytes to better understand conflicting results obtained from two laboratories (Green and Kalra (6), Ro et al. (1). When subpopulations of intact erythrocytes obtained by two different separation methods (33% bovine serum albumin and Stractan II gradient centrifugations) were incubated with L-[methyl-3H] methionine at pH 7.2 and 37 degrees C, membranes from both reversible and irreversible sickle erythrocyte populations showed about half the [3H]methyl group incorporation than that observed in normal erythrocytes. In addition, this difference in the level of methylation between normal and sickle cells was maintained during the entire course of a 2-hr incubation utilizing S-adenosyl-L-[methyl-3H]methionine, the immediate in vivo methyl donor.