The hemodynamic characteristics of Nuclepore (NP) membrane filtration were evaluated from a pressure (P)-flow rate (Q) relationship of erythrocyte suspension obtained by the vertical-tube method (Nichol et al., 1951) with a slight modification. It became evident that the vertical-tube method was a more quantitative and simple method than the conventional steady flow perfusion method using a variable speed pump. The P-Q relationship of erythrocyte suspensions in the perfusion of NP membrane consisted of a smooth curve convex to the P-axis at low P tending to a straight line at high P. The extrapolated linear segments of P-Q curves of erythrocyte suspension did not converge to the same point on the negative Q-axis, that is, the value of the negative intercept increased as flow decreased. This observation indicated clearly an obstruction or plugging of the pores by erythrocytes. It was made clear that flow rate measured with NP membrane filtration was influenced not only by intrinsic red cell deformability but also by several different factors, namely, the different distribution of pore size of NP membrane, microdust contaminated inevitably in red cell suspension, total volume of cell suspension through NP membrane, stagnation or retention of erythrocyte suspension and the hematocrit value of suspension. In conclusion, the present study shows that the precise definition of deformability depends on the method or rheological conditions, such as hematocrit value of suspension, used to measure it, even if confined within NP membrane filtration.