To determine the predictive value of a radiolabeled antiglobulin test for platelet crossmatching to select platelets for transfusion, we evaluated 230 transfusions in 42 alloimmunized oncology patients. Transfusions were classified according to the degree of HLA match. The value of the crossmatch in predicting platelet transfusion outcome was analyzed for each HLA category. For predicting the 1-hour corrected platelet count increment of all transfusions, the sensitivity of the test was 86 percent, specificity 84 percent, and total predictive value 84 percent. The crossmatch technique had similar predictive ability in all categories of HLA match, whether analyzing the results on the basis of the 1- or 24-hour platelet count rise.