The loss of sialic acid was determined in human platelets stored during a seven day period in their homologous plasma. Approximately 30% of the sialic acid was lost during the first three days of incubation at room temperature and a total of 73% was lost after seven days. The rapid in vitro loss of sialic acid may mimic a slower in vivo loss. It was found that platelets with a greater density had a higher sialic acid content than the less dense platelets. The loss of sialic acid from stored platelets could be completely inhibited by the addition of silyl compounds to the incubation plasma. The trisaccharide, N-acetylneuramin-lactose, gave a greater degree of protection than fetuin at comparable concentrations.