The American National Red Cross has initiated a cooperative clinical study designed to evaluate the therapeutic effectiveness of fresh platelets and platelets stored at 4 or 22 C for up to 72 hours. Preliminary results, in confirmation of earlier work, indicate an inverse relationship between platelet count and bleeding time in patients with thrombocytopenia transfused with fresh platelets. Also, the observed rise in platelet count immediately following transfusion of fresh platelets is generally maintained for four to eight hours after which time it declines significantly.