The ancillary tool according to the invention comprises a handle for manipulating an acetabulum provided, in its distal part, with a head for gripping the acetabulum and, in its proximal part, with a surface for application of a force of impaction. The tool further comprises at least one added endpiece adapted to be removably connected to the distal end of the handle. This endpiece defines both a face for wedging the acetabulum and an opposite face for interaction of the endpiece with the head of the handle, these faces both being borne by a radially deformable supple ring. The invention is particularly applicable to the positioning of an acetabular prosthesis in an anatomical or prosthetic cavity of a patient's hip.