To determine if platelet aggregometry was useful in selecting compatible platelet donors, six patients who had become refractory to random platelets were studied. Serum from the patient was added to citrated platelet-rich plasma from the prospective donor in a standard aggregometry system. Serial aggregometry studies revealed no platelet aggregation unless the refractory state had been reached. At that time positive aggregation occurred only between the poorly matched pairs. A correlation between platelet aggregation and HL-A histocompatibility was noted. Family members with negative aggregation were selected as donors, and their platelets were able to provide consistently satisfactory increments in the platelet count of the recipient who was refractory to random donors. In contrast, platelets from family members who exhibited positive aggregation failed to do so. These findings suggest that platelet aggregometry can be used to select compatible platelet donors.