Fourteen polymeric compounds were assessed for their abilities to protect human platelets from injury during freezing and thawing. The effect of polymer concentration and molecular weight, cooling rate, platelet concentration, and short-term storage on platelet recovery were studied. Polyvinylpyrrolidone, average molecular weight 10,000, was found to provide the greatest degree of protection to platelets. Recoveries of platelets of from 60 to 80 per cent were achieved using a 5 per cent final concentration of polyvinylpyrrolidone and a cooling rate of 8 C per minute. Methods of freezing small aliquots of platelets and platelet concentrates were developed. The frozen-thawed platelets were viable in vitro as shown by osmotic reversal, clot retraction, and aggregation by collagen and adenosine diphosphate. The techniques described could be developed readily to facilitate the long-term storage of platelets.