A disposable nozzle for use on the discharge end of the barrel of a needleless injector is threaded onto the front end of a holder mounted in the barrel. A resilient detent extending outwardly at an acute angle from the inner wall of a recess in the rear end of the nozzle repeatedly enters and exits notches in a sleeve on the holder when the nozzle is mounted on the holder providing an audible signal that the nozzle has not been used. Reverse rotation of the nozzle during removal from the holder results in breaking of the detent. Thus, mounting of a used nozzle on the holder would not be accompanied by a signal that the nozzle is unused.
