A prospective study was undertaken to assess the values of platelet aggregometry, lymphocytotoxicity, and mixed lymphocyte cultures in selecting compatible donors for patients refractory to random platelet transfusions. Donors were selected at random without regard to HLA types. Concurrent with each platelet transfusion, platelet aggregometry and lymphocytotoxicity were performed using patient serum and donor cells. The results were compared with HLA types, MLC, and increments in platelet counts. Forty-one transfusions were given to 21 patients; 27 were from related and 14 from unrelated donors. Platelet aggregometry was used successfully in selecting compatible donors in 37 cases (90%) with three false negative and one false positive results. Lymphocytotoxicity was useful in 73 per cent of cases with eight false negative and three false positive results. The response to platelet transfusions correlated poorly with HLA matches or MLC reactions. These data suggest platelet aggregometry and lymphocytotoxicity are useful cross matching techniques in selection of compatible platelet donors.