Results of a comparative study of the functional capacity of young versus old platelets are presented. A girl with thrombopoietin deficiency, who predictably produced platelets in response to plasma transfusion, was used as platelet donor. Her platelets obtained 4 days after infusion were used as young test cells. Platelets obtained at 18 or 21 days after infusion were used as old cells. Young platelets were found to be associated with normal bleeding times, normal clot retraction, normal or increased platelet adhesiveness, normal aggregation to ADP and collagen, and normal platelet factor 3 availability. Old platelets were found to be associated with long bleeding times, decreased platelet adhesiveness in vivo and in vitro, and deficient platelet factor 3 availability. The clot retraction and aggregation to ADP and collagen of old platelets, however, was normal. No differences between young and old platelets were observed by electron microscopy.