Two to 5 U of platelet concentrate were frozen together in single bags using glycerol-glucose as cryoprotective agents and a special freezing plate which produced a reproducible, rapid freezing rate when immersed in liquid N. After thawing there was no loss in platelets compared to prefreezing controls. Phase microscopic evaluation after thawing demonstrated large numbers of severely damaged platelets and a significant decrease in morphology score. Total ATP levels fell to 37-52% of controls, and thawed platelets did not aggregate with ADP or collagen or undergo release of nucleotides following incubation with thrombin. In vivo recovery 1 h after transfusion of autologous platelets administered to patients with leukemia was significantly inferior (P < 0.025) to results achieved in the same patients with autologous platelets frozen with dimethylsulfoxide. Significant damage occurs following freezing and thawing utilizing glycerol-glucose as cryoprotective agents for frozen platelets and further investigation is required prior to their clinical use. Blood banks currently interested in platelet cryopreservation for clinical use should utilize dimethylsulfoxide as a cryoprotective agent.