The in vivo viability and functional integrity of filtered platelets were compared with those of nonfiltered platelets in a controlled study. On two occasions, after template bleeding time, 14 healthy volunteers underwent plateletpheresis and received 600 mg of aspirin. Autologous 111In-labeled platelets were transfused without further manipulation (control) on one occasion and after filtration on a second occasion. The filter was primed and flushed with a buffered 12.6-percent solution of ACD-A in 0.9-percent normal saline (pH 6.5). After transfusion, the bleeding time was measured at 1, 4, and 24 hours and platelet survival at 10 minutes; 1, 4, and 24 hours; and daily for 6 days. The decrease in the bleeding time was not significantly different from that after transfusion of nonfiltered platelets (p greater than 0.2). Filtering of platelets did not affect 1-hour in vivo recovery (filtered, 69.5%; nonfiltered, 66%: p = 0.56) or the platelet survival (filtered platelet t1/2 = 83.0 hours, nonfiltered platelet t1/2 = 82.9 hours: p = 0.96). It can be concluded that filtration does not adversely affect in vivo recovery, survival, or functional integrity of platelets.