With the use of micromanipulation techniques, the shear modulus or “rigidity” mu, the recovery time tc, and the unfolding time tf for individual sickle cells have been measured at different oxygen tensions, temperatures, and cell densities. In these experiments, the partial pressure of oxygen was varied from 156 to 40 mm Hg and the temperature was controlled at 25 degrees C or 37 degrees C. Three mean cellular hemoglobin concentrations were studied: 29 g/dL, 33 g/dL, and 46 g/dL. The lighter cells (29 and 33 g/dL) exhibited at most a threefold increase in rigidity as the pO2 was decreased from 156 to 40 mm Hg. At 25 degrees C, the densest cells (46 g/dL) also exhibited a threefold increase. However, at 37 degrees C, the rigidity of these cells increased eightfold between 156 to 40 mm Hg. Compared with normal cells, this gives a rigidity that is 18 times larger. In contrast to the values for mu, the values for tc and tf remained essentially unchanged (within the accuracy of the experiments) for the lighter cells and could not be measured for the densest cells.