The results of most filtration assays for deformability of erythrocytes do not distinguish whether the entire population or only its small fraction exhibits abnormal rheological properties. We developed a simple filtration method for determination of the percentage of nonfilterable cells in erythrocyte suspension using membrane filters with mean pore diameter of 3.1
$\mu$
m. This method makes it possible to detect even minor abnormal subpopulations in erythrocyte suspensions. The flow rate of buffer depends on the number of free pores of a filter. The plot of the number of pores clogged by nonfilterable cells vs the total number of erythrocytes that were allowed to pass through the filter had a linear portion, with a slope representing the relative content,
$Z$
%, of nonfilterable cells in the suspension. We determined
$Z$
% for various medium osmolalities
$u$
and used the data to derive the distribution of erythrocytes in
$u_{\mathrm{cr}}$
(
$u_{\mathrm{cr}}$
is the maximum value of
$u$
at which an erythrocyte cannot pass through a pore of a given filter because of geometric limitations). The distribution of
$u_{\mathrm{cr}}$
in suspension of normal erythrocytes has a maximum of about 200 mOsm/kg and a half‐width of about 20 mOsm/kg. The distributions of
$u_{\mathrm{cr}}$
are altered in normal erythrocyte suspensions at decreased pH values, in cryopreserved and ATP‐depleted erythrocyte suspensions and in erythrocytes from a xerocytosis patient.
