Abstract: A jig for positioning an instrument, for example a biopsy needle, includes a stop (7) for defining the position of a free end (3) of the instrument in a first coordinate direction (Z) of a three-dimensional system of coordinates, and an adjusting mechanism (15, 17) for adjusting the position of the instrument in second (X) and third (Y) coordinate directions of the system of coordinates. The adjusting mechanism is accommodated in a housing (23) in which furthermore a phantom (5) is rigidly mounted. The adjusting mechanism includes a first and a second circular disc (15, 17), having a first and a second central axis (19, 21), respectively, each central axis extending perpendicularly to the plane of the relevant disc and through its center (16, 18). The first disc (15) is journalled in the housing (23) so as to be rotatable about the first central axis (19) and is provided with an eccentrically situated circular opening (27) whose diameter is substantially equal to the diameter of the second disc (17).