A drill guide assembly for determining the axis for drilling a bore to receive a component of an orthopaedic joint prosthesis, includes a drill guide having a sleeve and a bulb at one end of the sleeve. A frame, which can be fastened to the bone, includes a housing which defines a recess in which the drill guide bulb can be received with the drill guide sleeve extending out of the recess, in a direction away from the bone, so that the angular orientation of the drill guide sleeve relative to the housing can be adjusted by movement of the bulb within the recess. The drill guide can be locked relative to the housing against angular adjustment by a clamp which comprises a lower pair of clamping surfaces provided by the drill guide bulb and the internal wall of the recess respectively, and an upper pair of clamping surfaces on the drill guide and the housing respectively, arranged so that the drill guide can be locked against angular adjustment by engagement between the frame clamping surfaces and the drill guide clamping surfaces of each of the lower and upper pairs, in which the upper clamping surface of the drill guide is spaced apart from the bulb along the drill guide sleeve.