The effects of a new micropore transfusion filter (Fenwal 4C2423) on stored whole blood have been examined. Five filters were preloaded by passage of two units of outdated type specific bank blood, and the effects of filtration on a third unit, consisting of 21-day-old blood, flowing under 150 mmHg pressure, were measured. Filtration did not significantly alter red blood cell count, total hemoglobin, red blood cell fragility, plasma sodium, potassium, albumin, or globulin. Some platelets and white blood cells were removed and a small amount of hemolygis of erythrocytes (less than 0.1%) was observed. Removal of microaggregates, assessed by Coulter counting, screen filtration pressure, total screen porteins, wet and dry weights of material retained, and scanning electron microscopy, was shown to be excellent over the entire range of particle size. Comparison of the Bentley PFS-127, Fenwal 4C2417, Johnson & Johnson Intersept, Pall Ultipore, and Swank IL200 filters led to the conclusion that the Fenwal 4C2423 was both a significant improvement over the previous Fenwal design and comparable to the most efficient of these filters for both the removal of microaggregates during massive blood transfusion and for the blood flow rates obtained.