An instrument for locating and sealing a previously formed percutaneous puncture in a wall of a blood vessel or lumen of a living being. The instrument (20) has a distal end for insertion through the puncture into the blood vessel or lumen and a proximal end for location outside the body of the living being, and comprises a closure means (22) for closing said puncture (24), and a locating means (28, 200, 300, 400, 402,502) for locating the wall of the blood vessel or lumen (26). The vessel locating means comprises a flow passageway (10, 402, 502) extending from an inlet (204, 304, 404, 506) of the passageway adjacent the distal end (28C) of the instrument (20) to the proximal end of the instrument (20). The passageway being constructed so that in use of the instrument (20) when the instrument is in a first position relative to the wall of the blood vessel (26), the inlet (204, 304, 404, 506) of the passageway is located inside the blood vessel to allow blood from the interior of the blood vessel to enter into the passageway and flow through the passageway to become visible at the distal end of the instrument (20). When the instrument (20) is in a second position relative to the wall of the blood vessel or lumen (26), the inlet (204, 304, 404 506), the passageway is obturated by the wall of the blood vessel thereby to prevent blood from the interior of the blood vessel from entering the passageway.