PURPOSE: To preserve platelet safely for a long time and supply to a human body when required, by mixing specific wt.% citric acid ester with specific wt.% polyvinylchloride.

CONSTITUTION: A platelet preserving bag consists of a compound obtained by mixing 20-80 pts.wt. citric acid ester with 100 pts.wt. polyvinyl chloride. Besides them, as material to bag, stabilizing agent, such as calcium or zinc, gliding agent, such as steric acid epoxidated soya bean oil, etc., may be added, if required. Formation of the bag is performed in such steps as mixing a plasticizer with polyvinyl chloride uniformly using a blender or mixer, melting the resultant mixture, intruding into a die in the form of medical bag, followed by blow shaping or extrusion into sheet form, and thereafter heat sealing the end part. Thus, a medical bag is yielded. The thickness of the sheet may be any selected depending upon the intended use, example being 200-500μm.