Red blood cell (RBC) deformability was determined with an ektacytometer in fractions separated on the basis of differences in cell volume or density. Deformability was measured with ektacytometry (rpm-scan and osmo-scan). We studied three groups of RBC fractions:1. By counterflow centrifugation we obtained fractions of different cell age which showed a slight decrease in mean corpuscular haemoglobin concentration (MCHC) and an increase in surface-to-volume (S/V) ratio in fractions with older cells. 2. By Percoll fractionation fractions were obtained which showed a pronounced increase in (MCHC) but no change in S/V ratio. 3. By a combination of both fractionation techniques, fractions were obtained which showed an increased MCHC and an increase in S/V ratio. Deformability in group 1,2 and 3 showed respectively no change, a moderate decrease and a pronounced decrease in fractions of older cells. A decline in deformability occurs during the aging process of the red blood cell. This decline in deformability in old red cells is greater than originally thought. This decline is the result of an increase in haemoglobin concentration and a second factor, probably a decrease in membrane elasticity.