Abstract: A blood vessel clamp has a pair of arms (2,4) each supporting a respective jaw (8,12). The arms are pivotally interconnected by a box joint (26) formed by an intermediate portion (22) of one arm (2) extending through a slot in an intermediate portion of the other arm (4) and imprisoned therein by a pivot pin (28). The internal wall of the slot is provided with a protrusion (30) at a location remote from the pin (28) such that as a part (32) of the intermediate arm (22) enters the slot during the last stages of closure of the jaws (8, 12) it engages the protrusion (30) and forces the opposite walls of the slot apart against their own resilience. This creates an increase stiffness in the joint (26) so that the jaws will remain in any desired position until a predetermined force is applied to over the stiffness whereupon they can be moved to a new position.