An instrument for locating the cutting plane on the head of a long bone in a surgical procedure, wherein the head of the bone is replaced with a joint prosthesis component, includes an axial reference shaft which can be arranged parallel to the axis of the bone and a cutting guide which can be moved relative to the axial reference shaft. The cutting guide has a reference block for location against the bone relative to the edge of the bone at one side and at least one arm which is shaped so that it can extend from the cutting guide over the face of the head. The arm can be located positively with respect to features on the head so that the location of the reference block along the axis of the bone can be set with reference to features on the head of the bone which are contacted by means of the arm.
