The effects of a new transfusion filter (Bentley PFF-100) on stored whole blood have been examined. Six filters were preloaded by passage of two units of outdated type-specific bank blood and the effects of filtration on a third unit of 21-day-old blood flowing under 19.95 kPa (150 mm Hg) pressure were measured. Filtration did not significantly alter erythrocyte or leukocyte count, total or plasma haemoglobin, red cell fragility, plasma sodium, potassium, albumin, or globulin. Platelet counts were reduced by 31%. Removal of microaggregates, assessed by Coulter counting, screen filtration pressure, total screen protein, wet and dry weight of material retained and scanning electron microscopy, was shown to be effective over the entire range of particle size. In comparison with other transfusion filters previously examined in this laboratory, the Bentley PFF-100 filter combines the feature of moderately efficient microaggregate removal with rapid blood flow rate. Compared to its predecessor, the Bentley PF-127, this new filter design has improved flow characteristics without loss of efficiency of microaggregate removal.