A coaxial dual flow catheter has been tested for thrombapheresis with a continuous flow blood cell separator in order to render the procedure as comfortable as possible for our donors. Therefore, two groups of five donors underwent a standard plateletpheresis: 3500 ml of blood were withdrawn at a speed of 35-45 ml per minute and were anticoagulated with ACD-A in a 1/9 ratio to whole blood. In the first group one vein puncture was effectuated with the coaxial catheter after local anesthesia. Two 14G catheters were used in the second group. In both groups the platelet yield was studied as well as the half life of 111-Indium-oxinate labeled platelets after autologous transfusion. Although the comfort of the donors was greatly improved, the one needle procedure resulted in a mean platelet yield of 1,28.10 while the two needle procedure yielded 3,46.10 platelets. In contrast, the survival of the platelets after autologous transfusion did not differ significantly in the two groups: t 1/2 in group I was 3,94 d, in group II 3,66 d. It is advocated that the poor efficacy of the single needle procedure might be due to recirculation.