To study how to control the ammonia level in platelet concentrates during storage, the effects of storage conditions on ammonia accumulation in the product were examined The ammonia level increased significantly with increasing storage temperature from 4℃ to 37℃ and reached approximately 500μmol/l after 72 hours of storage at 22℃. There was a negative relationship between the ammonia level and pH value of the product. The ammonia accumulation due to a fall in pH was depressed by applying a gentle flat-bed mode of agitation during storage. More vigorous agitation, such as elliptical agitation, resulted in higher ammonia accumulation. Analysis of changes in amino acids of the product showed that ammonia was mainly derived from non-enzymatic hydrolysis of glutamine in plasma. Glutamic acid had a preventing effect on the ammonia accumulation. The effect was dose-dependent and required approximately 50mg/dl final concentration to reduce the ammonia level to about half of the normal value. I consider the addition of glutamic acid to the blood preservative to be useful in reducing transfusion reactions due to ammonia.