a method of preparing a medical bag comprising a pair of heat - sealable soft plastic sheets which are heat - sealed at predetermined heat - sealing peripheral portions thereof , comprising the steps of preparing the pair of heat - sealable soft plastic sheets ; coating a 0 . 05 to 1 . 5 % by weight solution of a silicone resin composition on one substantially entire surface of each of said sheets , the silicone resin composition having at least partial crosslinkability ; at least partially crosslinking the silicone resin composition so as to form substantially non - flowable , substantially continuous layers each of which covers the one substantially entire surface of each of the sheets ; and superposing said pair of heat sealable sheets with the respective side of each sheet coated by the non - flowable silicone layer in face to face relationship and heat - sealing the peripheral portions of the sheets to provide said medical bag .