In general purpuric bleeding appears at higher platelet levels when thrombocytopenia develops rapidly than when the process is chronic. Thrombocytopenic bleeding may be controlled by the transfusion of viable platelets but quantitative evaluation of effectiveness now is very difficult. Measurement of the life span of transfused platelets may be linear or exponential but the ideal method for performing such studies has not been described. In vitro measurement of platelet functions does not serve as a useful index of clinical effectiveness. Platelet transfusions are clinically useful and the criteria of effectiveness are platelet counts and cessation of external bleeding. In the last analysis judgment of the value of platelet transfusions is largely based on clinical experience.