Abstract: A rotary knife holder for a longitudinal slitter in which a rotary knife disc 8 is displaceable relative to a second rotary knife disc 10, co-operating therewith, by separate pneumatic cylinders 12 and 14 to which air is supplied by a common controllable feed line 26 against a resilient restoring force initially radially and then axially. To achieve instant response, to permit the feed speed and feed force for the cylinder 14 to be as independently adjustable as possible and, in addition, to avoid any slip-stick effect in the axial feed movement, an adjustable reducing valve 38 is connected prior to the cylinder 14. This reducing valve 38 is double-acting and is filled with a damping fluid which, during the feed stroke, is expelled through a throttle member 50 into an air vessel 52.