Filter columns (Imugard® filters) packed with cotton wool prepared from Gossypium barbadense cotton removed 95-100% of leukocytes from packed red cell suspensions. Recovery of red cells was about 95%. The efficacy was similar for ACD and heparinized blood. Blood was filtered below 10°C within 30 min under pressures of less than 0.2 kg/cm2. The shape and functions of the red cells were not altered by filtration. No pyrogenic or toxic substances, and little particulate matter were eluted from the filters.