Changes in filterability during 28-day bloodbank storage of normal, sickle trait, and sickle cell anaemia red cells were investigated. The technique used involved constant positive-pressure filtration of red cell-saline suspensions through cellulose nitrate membrane filters of 8-microns pore size for 2 min. Erythrocyte filtration rate was expressed as the number of red cells filtered per minute; calculated from the volume and red cell count of the filtrates. This was a departure from the commonly used filtrate volume measurements alone, and seemed to permit a clearer definition of changes in filterability during storage of red cells. It was found that changes in filterability during storage followed an exponential pattern for normal and sickle trait red cells but not for sickle cell anaemia cells. Filterability is known to correlate well with deformability which in turn is an important determinant of in-vivo survival of red cells. It may therefore be concluded that sickle trait red cells do not manifest any peculiar deformability or other changes during storage which might affect their post-transfusion in-vivo survival more adversely than normal red cells.