Filtration experiments on red blood cell suspensions are usually conducted in a saline buffer solution. As a result, the flow of a particle in a pore is largely dominated by viscous effects, and it is not possible to distinguish between normal and membrane altered cells. A new approach to red cell filtration is proposed here, whereby the cells are suspended in a Dextran solution that has roughly the same viscosity as the internal hemoglobin solution. It is thus aimed to detect alterations of the membrane properties. In order to prove this point, filtration measurements are conducted with a hemorheometer on dilute (hematocrit 8%) suspensions of normal and membrane hardened (diamide treatment) cells, suspended either in a 8 mPa.s Dextran solution or in a 1 mPa.s buffer solution. As expected when the cells are suspended in buffer, there is no detectable difference in the filtration index for normal and treated cells. However, when they are suspended in the 8 mPa.s solution, the filtration index is significantly larger for treated than for normal cells. This shows that filtration in a viscous liquid can be used to measure changes in cell deformability due to membrane modifications.