We compared the reactivity of fresh platelets with platelets frozen in dimethylsulfoxide (DMSO) or without a cryopreservative by testing agglutinating and nonagglutinating platelet-specific alloantibodies in titration. Frozen platelets, irrespective of the method of preservation, showed no loss of antigenicity as compared with fresh platelets. The yield of platelets frozen without a cryopreservative proved to be higher (80%) than platelets frozen in the presence of DMSO (50%). Platelets sensitized with autoantibodies from patients with autoimmune thrombocytopenia could also be stored by simple freezing in platelet-rich plasma and used for (re)testing.