115 patients with bone marrow aplasia/hypoplasia received a total of 567 transfusions of fresh HLA-selected platelet concentrates at random from the AS-104 and CS-3000 and, whenever possible, from both separators using the same donor. By daily platelet counting pre and up to seven days post transfusion, the posttransfusional increments per 10(11) platelets transfused were calculated. Fresh platelets collected from the AS-104 showed comparable in vivo recovery at the first day post transfusion but significantly better survival compared to those from the CS-3000. This is in line with in vitro studies published before, where better in vitro function and morphology were observed. Increased platelet yields and improval of the platelet survival of the PC's from the AS-104 should result in prolonged transfusion intervals. When additionally evaluating a limited number of PC's from the AS-104 stored in teflon bags up to five days before transfusion (n = 12), our results were not favorable compared to PC's from the CS-3000 stored in polyolefine bags. As this seemed to be due to the geometry of the bags, this was consequently changed.