Red blood cells exposed to cyanate (CNO) in vitro have a concentration- dependent decreased cell survival time associated with an inhibition of the ability of the cell membrane to synthesize lipids. The t1/2 of rabbit erythrocytes exposed to 30 mM or 50 mM cyanate for 1 hr at 37 degrees C is reduced from the normal 24 days to 15 and 9 days, respectively. The cyanate-induced defect in membrane lipid metabolism is irreversible. Carbamylation of membrane proteins and damage to metabolism are minimized by limiting exposure in vitro to 15 mM cyanate at 4 degrees C for 30 min. Cells carbamylated under these conditions do not have a shortened life span. Levels of globin carbamylation of 0.5 moles CNO/mole hemoglobin, shown to be clinically effective in prolonging the life span of sickle erythrocytes, are obtained under these conditions and reach maximal levels after only 30 min of incubation. Carbamylation of blood in CPD anticoagulant is inferior to either ACD or heparin. The findings indicate that adequate carbamylation of sickle erythrocytes with minimal red cell membrane damage can be achieved without significant modification of the standard plasmapheresis procedure utilized by the working blood bank.