A cutting tool holding assembly is arranged for rapid replacement of cutting tools on machine tools. The assembly includes an adaptor (12, 14) for locating a tool holder (16) in a drive spindle (10). The adaptor has a base member (12) which is secured in the spindle (10) and a body member (14) which is secured releasably to the base member (12). The tool holder (16) is secured to the body member (14). The base member (12) is secured in the base of a bore (19) in the spindle (10) and the body member (14) is releasably secured against rotation relative to the spindle (10).
