Anaphylatoxins generated during storage of platelet concentrates (PCs) may potentially have side effects on platelet transfusion. We evaluated the anaphy- latoxin-scavenging abilities of white blood cell reduction filters. Among the commercially available filters for PCs, one made with polyester fiber (PL50) dramatically adsorbed C3a and C4a anaphylatoxins to the respective mean level of 1,721-208 ng/ml and 1,240-141 ng/ml in 3-day-old PCs. C3a and C4a were measured as the native and des Arg form of each complement by radioimmunoassay. C3a and C4a anaphylatoxins in the supernatant plasma fraction from 3-day-old PC again decreased from 1,136 to 114 ng/ml and from 1,086 to 65 ng/ml, respectively. The filter also adsorbed 85% of platelet factor 4 (PF4) and 31% of β-thromboglobulin (β-TG), which had been released from platelets into the plasma during storage. The plasma levels of adhesive proteins such as fibronectin, fibrinogen, and von Willebrand factor, and plasma lactate dehydrogenase activity did not decrease after filtration. Another polyester filter (PL5A), on the other hand, significantly increased C3a and C4a levels with filtration. In addition, there was no PF4 adsorption ability during the filtration. The filters for red cells (RC50, BPF4, and R500A) had no anaphyla- toxin adsorption capabilities. The observed specific adsorption of anaphylatoxins might be attributed to the electrostatic force between the positively charged anaphylatoxins with high pi and the possibly negatively charged filter membranes. Since PF4 and ß-TG have positively charged moieties in the C-terminal position, the same adsorption mechanism might operate. We have obtained a useful scavenging filter for the evaluation of side effects of anaphylatoxins on patients who received old PCs.