A centralizer (1, 101) suitable for centralizing a tubular member within a substantially vertical bore, the centralizer comprising a main body (2, 102) adapted to be connectable around a tubular member to be centralized, the main body defining a longitudinal axis of the centralizer, and a plurality of longitudinally extending abutments (18, 118) spaced apart around the main body (2, 102), each abutment extending radially outwards from the main body for making abutting contact with said bore and at least one of said abutments having an adjustment mechanism (3, 103, 34, 134, 36, 136, 42) for making a radially adjustable abutting contact with said bore, said adjustment mechanism comprising a radially movable outer blade (34, 134) for making said abutting contact, a longitudinally movable wedging member (36, 136) located between the outer blade and the main body (2, 102) for moving the outer blade radially into said abutting contact, at least one guiding mount (3, 103) for guiding said radial movement of the outer blade, and at least one longitudinally extending ramp surface, said ramp surface being inclined with respect to said axis, wherein: - the outer blade (34, 134) is constrained to move in a substantially radial direction by said at least one guiding mount (3, 103); and - the wedging member and the outer blade are both relatively movable with respect to the main body and to each other and the wedging member is constrained between the outer blade and the main body (2, 102) to move in a substantially longitudinal direction, the wedging member and said at least one ramp surface being configured to engage with one another such that as the wedging member moves longitudinally, the wedging member causes the outer blade to move radially, characterized in that the arrangement of the wedging member (36, 136) between the main body (2, 102) and the outer blade (34, 134) is such that when the main body is oriented with the longitudinal axis being substantially vertical for centralizing said tubular member within said bore, the weight of the wedging member is sufficient to automatically cause the wedging member to drop relative to the outer blade and the main body, whereby, in use, the wedging member moves downwards between the main body and the outer blade in said substantially longitudinal direction under gravity and said engagement of said moving wedging member and said ramp surface causes the outer blade to move in said radially outwards direction for making said abutting contact, the movement and subsequent engagement of the outer blade with the bore thereby being gravity-driven. - the outer blade (34, 134) is constrained to move in a substantially radial direction by said at least one guiding mount (3, 103); and - the wedging member and the outer blade are both relatively movable with respect to the main body and to each other and the wedging member is constrained between the outer blade and the main body (2, 102) to move in a substantially longitudinal direction, the wedging member and said at least one ramp surface being configured to engage with one another such that as the wedging member moves longitudinally, the wedging member causes the outer blade to move radially,