Abstract: An enclosure member such as a medical isolator provides an isolated environment at a higher pressure than the ambient environment, and has a transfer port for passage of objects into and out of the enclosure member. The transfer port comprises a flexible sleeve having one end coupled to the enclosure member around an opening in the enclosure member and having its other end positioned externally of the enclosure member. An air permeable member is arranged across the opening in the enclosure member and passes a restricted, substantially non-turbulent flow of air along the sleeve. The air permeable member is movable away from the opening during passage of objects through the conduit, and may comprise a sheet of flexible material hanging in the form of a curtain across the opening. Preferably the cross-sectional area of the sleeve diminishes with increase of distance along the sleeve away from the opening of the enclosure member to produce an increasing air velocity along the sleeve.