A randomized study was performed in 54 thrombocytopenic patients with acute leukemia. Alloimmunization of recipients of random multiple-donor platelet concentrates (MD group) was compared to that of patients receiving random single-donor platelets (SD group). In the SD patients, formation of alloantibodies (mostly anti-HLA) occurred less frequently (p less than 0.002), after a longer time period (p less than 0.002), and after a higher number of transfusions (p less than 0.005) as compared to MD patients. SD patients also became refractory to random platelets less frequently (p less than 0.005), after a longer time period, and after a higher number of transfusions (p less than 0.02). In SD patients, the increments after the first and the last transfusion were in the same range, whereas in MD patients, the 1-hr (p less than 0.001) and the 24-hr (p less than 0.025) increments decreased from the first to the last transfusion. Thus, the use of random SD platelet transfusions postponed alloimmunization.