Red blood cell deformability has been studied by the initial filtration flow rate as a function of temperature. The well-known transition at 49-50 degrees C (probably due to spectrin denaturation) is shown. Another transition is demonstrated around 18 degrees C (the cell becomes stiffer below this temperature range). The erythrocyte membranes prepared by a mild dialysis technique have the same deformability as intact erythrocytes at room temperature; they also show the same low-temperature transition. No such transition has been found for hemoglobin solutions of viscosity 30 g X dl-1. It is interesting to compare these results with those obtained by other methods which measure the properties of natural or artificial lipid membranes and which also demonstrate a thermal transition at 15-20 degrees C. Therefore, the deformability of intact normal erythrocytes seems to depend mainly on the rheological properties of the membrane.