In order to determine the effect of concentration and pooling of platelet rich plasma (PRP) from multiple donors on response to platelet transfusion, the results of investigations with various preparations of fresh platelets for transfusion were analyzed. Transfusions of multiple donor, pooled PRP and platelet concentrates (PC) were compared by study of response to 385 transfusions in 55 patients with thrombocytopenia. The mean platelet increment an hour after PRP transfusion was 55,000/5 units/sq. meter surface area of the recipient. The mean fraction of platelets surviving in the circulation on the second day was 36 per cent of the increment. The results of the multiple donor pools were similar to PRP transfused from single donors. On the other hand, concentration of platelets reduced the effectiveness of platelet transfusion by over 50 per cent. Several methods of preparation of platelet concentrates were compared. The effects of age of the concentrate, resuspending medium, anticoagulant, and centrifugation are enumerated. However, a controlled clinical transfusion study showed that the decreased response to PC transfusions was due mainly to the deleterious effects of high speed centrifugation (1,500 to 5,000 × g) employed in the process of concentrating platelets from PRP. No method of preparing platelet concentrates was shown to be superior. Although inferior to PRP in terms of response to transfusion, concentrates are valuable because of their ease of handling and clinical utility when large quantities of platelets in a small volume of fluid are indicated