A workpiece-holding unit for installation on machining centers for connecting rods, comprising a crosspiece which is adapted to be rotatably supported about its axis in front of a machining head of the machining center and has internal, hydraulic longitudinal ducts which communicate with respective hydraulic outlets on the main faces of the crosspiece and are connected to hydraulic feed means. Several workpiece-holding tools are installable on the main faces of the crosspiece and are provided with positioning mechanisms and locking mechanisms for at least one connecting rod, which are hydraulically operated via respective hydraulic lines leading at positions aligned to said hydraulic outlets.