A medical instrument such as a catheter introducer comprises a housing, with the housing carrying a valve for receiving and sealing an elongated member with penetrates the valve. The valve comprises an elastomeric partition member which, in turn, comprises a first, solid hydrophilic elastomer wall to limit the passage of aqueous liquids while permitting the passage of water vapor through the wall. The wall defines outer and inner faces, with the outer face carrying a hydratable lubricant material, and the inner face being bonded to a foamed elastomer material. The elastomer material facilitates sealing of the valve when an elongated member is penetrating therethrough, and it facilitates penetration of the elongated member. Also, the foamed elastomer material serves as a support for the hydrophilic elastomer wall, which is typically very thin to permit a high rate of water vapor diffusion.