In a previous work we showed an alteration of erythrocyte filtration ability in patients with Alzheimer's disease according to their age and illness duration. This study has for aim to find a criteria of deformability that would be constant in all Alzheimer patients and would show a modification of red cell membranes. The erythrocyte filterability was studied in this present paper, in accord to Reid and Dormandy method using two values of depression (5 and 0 cm of water). These depressions correspond to the physiological values of blood pressure at the level of precapillary and capillary systems. The ratio between the result obtained at 5 cm and the result at 0 cm is constant in normal patient without organic disease and it is independent of age. At the opposite, this ratio increase very significantly in all Alzheimer patients, and this is not correlated to the initial value of filtrability. This ratio could be an index of the alteration of red cell membranes.