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Summary. Multiple units of platelet concentrate obtained by plateletpheresis of normal, ‘random’or HL-A matched donors were pooled and frozen in polyolefin bags using 5% dimethylsulphoxide (DMSO) as a cryoprotective agent and a controlled freezing rate of 1°/min. The platelets were stored at approximately — 120° C for as long as 201 days, thawed rapidly at 37°, washed once and resuspended in ACD plasma prior to transfusion. Two different final concentrations of platelets (˜ 2.7 and 9.0 × 1012/1.) were studied. Twenty-three thrombocytopenic patients have received a total of 40 frozen platelet transfusions. The mean freeze-thaw loss was 21% and was similar for both platelet concentrations. All transfusions were well tolerated and there were no side effects attributable to the small amounts of DMSO infused. Increments in platelet counts I h after transfusion ranged from 0 to 102 × 109/1. with an overall mean corrected increase in evaluable patients of 12 800 (increase × surface area (m2)/number of platelets transfused × 1011). Corrected increases tended to be greater with the low concentration of platelets. Overall, the increase in count for the frozen platelet transfusions was 65% of the increments obtained with fresh platelet transfusions administered within I week of the frozen platelets. Bleeding times were partially corrected after four out of six transfusions with post-transfusion counts greater than 50 × 109/1., and active haemorrhage was controlled in some patients by frozen platelet transfusions. These results indicate that pooled platelets can be frozen, thawed and transfused with reasonable efficiency. The frozen platelets can circulate and function haemostatically and may eventually play an important role in supportive care.