The effect of deoxygenation on deformability of sickle cells (Hb SS RBC) was studied quantitatively by a microsieving technique with the use of polycarbonate filters having micropores of 5 mum in diameter. The rate of pressure rise and the relative resistance during filtration through polycarbonate sieves increased progressively when the P02 of Hb SS RBC suspensions was reduced below 80 mm. Hg. Control studies on Hb AA RBC did not show any significant change in either parameter when deoxygenated. The present study on the filterability of Hb SS RBC and our previous data on the viscometric behavior of Hb SS RBC serve to establish the quantitative relation between P02 and the deformability of Hb SS RBC.