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 < 0.002), after a longer time period (P < 0.002) and after a higher number of transfusions (P < 0.005) as compared to MD patients. SD patients also became refractory to random platelets less frequently (P < 0.005), after a longer time period and after a higher number of transfusions (P < 0.02). In SD patients, the increments after the 1st and the last transfusion were in the same range; in MD patients, the 1 h (P < 0.001) and the 24 h (P < 0.025) increments decreased from the 1st to the last transfusion. The use of random SD platelet transfusions postponed alloimmunization.