Although HLA‐matched platelet transfusions are of value in the support of some alloimmunized thrombocytopenic patients, poor posttransfusion increments can be observed following HLA‐matched platelet transfusions and conversely good posttransfusion increments may result after HLA‐mismatched platelet transfusions. We have explored the possibility that in vitro assays in addition to HLA typing might better select compatible donors for refractory recipients. Our studies suggested that a platelet migration inhibition assay is predictive of platelet transfusion responses in HLA‐compatible and incompatible donor‐recipient pairs, while lymphocyto‐toxicity is predictive of posttransfusion increments only in HLA‐incompatible donor‐recipient pairs. Granulocy‐totoxicity, microleukoagglutination, and capillary leu‐koagglutination showed no value in predicting platelet transfusion increments, either in HLA‐compatible or incompatible