Abstract: A tool holder for easy and quick insertion in and removable from a high speed spindle of a machine tool. The tool holder is positively driven by the spindle by means of a quickly engagable and disengagable clutch. A drive adapter is insertable in the bore of a conventional spindle and has a drive clutch element which is disengagably connected with a complementary drive clutch element of the tool holder. The tool holder also has a resilient, flexible means in the form of an O-ring which frictionally engages the interior of the spindle so as to prevent inadvertent removal of the tool holder when reverse rotational forces are applied on the tool or at lower speeds of the spindle. The resilient, flexible means is centrifically urged in tight frictional engagement with the spindle when the latter is operating.