The viability of both autologous and donor platelets collected by means of a semi-continuous flow centrifuge (Haemonetics Model 30) was measured. Radiochromium labelled pooled platelets harvested from six passes and reinfused into their normal donors showed recoveries and survivals of 58%± 8 and 9.6 d ± 0.8 compared with normal values of 65%± 5 and 9.5 d ± 0.6, respectively. When the viability in aplastic thrombocytopenic recipients of platelets collected from normal donors by the Haemonetics Model 30 was compared with that of platelets collected from the same donors by multiple bag plateletpheresis the results were very similar: recoveries of 82%± 18 and 80%± 25 and survivals of 3.9 d ± 2.0 and 3.8 d ± 1.3, respectively. The in vitro function of the collected platelets in response to aggregating agents was normal when measured within 60 min of collection at pH 7.4. The relationship between bleeding time and post-transfusion platelet count in aplastic, thrombocytopenic patients showed that in vivo function was maintained.