A closing mechanism for the mold halves of an I.S. machine, including two yoke-like mold holders mounted on parallel guides and movable between a closed position and an open position by means of a drive. A spindle transmission is provided laterally to at least one guide, the spindle transmission including a threaded spindle that is connected to the drive via a transmission and is in engagement with two threaded sleeves, each of which is connected to one of the two mold holders via two threaded sections running in opposite directions. By means of the drive, a servomotor, the opening and closing movement of the mold holders is carried out. Two pneumatically actuated piston-cylinder units are provided in order to apply the clamping force required between the mold holders or the mold halves in the closed position.