Irreversibly sickled cells (ISCs), considered by some to be of major pathophysiologic significance, have been reported to comprise between 5-50% of the total red cell population in patients with homozygous sickle cell anemia. Since the deformation of erythrocytes containing sickle hemoglobin is highly dependent on the concentration of hemoglobin in the deoxy conformation, any method established to enumerate the true ISC count requires the hemoglobin to be in the full oxy or liganded conformation. Because the oxygen dissociation curve for sickle erythrocytes is significantly shifted to the right, extremely high partial pressures of oxygen are required to approach full saturation. On the other hand, fully liganding the hemoglobin in the oxy conformation with carbon monoxide (CO) can be readily accomplished. We found that there is a significant reduction in the average number of sickled forms in the peripheral blood of sickle cell anemia patients after incubation in CO (to a value of 6.5 +/- 3.5%) when compared to conventional methods for ISC preparations. These results suggest that fully liganded erythrocytes should be used in quantitating ISCs in studies of the pathophysiology of this disease, especially since ISCs are likely to affect rheology differently from reversibly sickling cells.