a flexible bag formed from at least one plastic sheet folded to form a tube and provide overlapping , longitudinally - extending margins , is disclosed . the overlapping margins are joined by a series of longitudinally - spaced bonds , typically heat seals . a membrane permeable to sterilizing vapor and which may comprise a strip of paper having high wet strength is bonded to the tube and overlies the portion of the tube including the overlapping margins . the longitudinally - spaced heat seals preferably secure the overlapping margins to the membrane as well . the various heat seal patterns for joining the overlapping margins and membrane , as well as particular embodiments of the bag for retaining liquid and dry contents , are also disclosed . the sealing arrangement of the invention provides a bag capable of holding , even under autoclaving conditions , bulky , heavy materials which would otherwise rupture the paper strip or the seals joining the strip to the plastic tube .