Fresh platelet concentrates were given to 18 adult patients on 68 occasions without adverse effect. The concentrates were prepared by means of a closed‐system plateletpheresis technic which permitted rapid and frequent bleeding of donors. There was a direct relationship of the amount of platelets infused to the mean posttransfusion increment. Excellent hemostatic effect was noted in 84% of patients with active bleeding when the platelet increment was 40,000/mm3or better. Transfusion of large numbers of platelet concentrates is effective in controlling or averting bleeding in adults with thrombocytopenia and is recommended on a therapeutic and prophylactic basi