Abstract: An apparatus for capping and uncapping containers includes a framework for storing screw-on caps in a number of stacked columns. A guide for aligning containers, such as bottles or vials held in a standard holder, consists of two members movable with respect to one another and defining a number of openings of adjustable size into which the containers are to project so as to become aligned with the stacked caps. An endless, cogged belt is operatively positioned within grooves formed on longitudinal surfaces of the members, portions of the cogged belt projecting into the openings defined by the members into which the containers are to project. This cogged belt in operation contacts the exterior surface of screw-on caps and turns them with respect to the threaded necks of the containers on to which they are to be placed or from which they are to be removed. The two members can be moved apart relative to one another in order to allow the necks of the containers and the caps to be positioned within the openings.