Abstract: Motor-vehicle bodies or sub-assemblies thereof are assembled in a welding-assembly station with the aid of frames arranged at the two sides of the conveying line that traverses the station, said frames being provided with fixtures for locating and clamping the various parts of the body. The side frames are moved between their operative position in the station and an inoperative position at a distance from the station with the aid of multi-axis manipulating robots. Said robots, however, do not carry the frames up to their final operative position for assembling the body, but rather unload them on two rigid structures, which are prearranged at the two sides of the line and are movable in a transverse direction with respect thereto between an open condition and a closed condition. The movable structures, once they have received the frames from the robots in their open condition, displace into their closed condition, thus carrying the frames into their final operative position.