Platelets frozen in glycerol-glucose frequently aggregated on thawing or reconstitution. This has been eliminated by using a nonplasma diluent (pH 6.5) and a new freezing bag. The results were improved platelet 14C-serotonin uptake and readily demonstrable aggregation to ADP, epinephrine, collagen and thrombin. 14C-serotonin uptake was at maximum when platelets were frozen at thrombocrits of up to 15%. Hemostatic effectiveness was demonstrated in a severely thrombocytopenic patient with acute myelogenous leukemia in relapse, who was supported entirely with two autologous frozen platelet transfusions for the 13 days before bone marrow activity resumed.