Hemolysis in paroxysmal nocturnal hemoglobinuria (PNH) is considered to be a result of an intrinsic membrane defect. This defect may result in abnormal material properties of PNH erythrocytes. To examine this hypothesis, fragmentation failure, and membrane deformability were assessed in the absence of complement by micropipette techniques. Membrane viscosity was determined by observing relaxation of deformed cells. Results show a bimodal distribution of force for membrane failure, membrane viscoelasticity, and elastic shear modulus. One population requires significantly less force for fragmentation, mean 0.56 X 10(-6) dyne; has increased membrane viscosity, mean 0.205 X 10(-2) dyne sec/cm; and has decreased elastic shear modulus, mean 0.56 X 10(-2) dyne/cm. A second population resembles control with fragmentation force, mean 1.19 X 10(-6) dyne, control 1.05 X 10(-6) dyne; membrane viscosity, mean 0.112 X 10(-2) dyne/cm, control 0.102 X 10(-2) dyne sec/cm; elastic shear modulus, mean 0.70 X 10(-2) dyne/cm, control 0.78 X 10(-2) dyne/cm. The percent of cells with abnormal material properties corresponds to the percent of PNH III cells determined by complement lysis. Thus, the hemolysis attributed to an abnormal clone of erythrocytes in PNH is associated with an intrinsic membrane abnormality which predisposes to lysis.