A system for protecting an individual against injury or infection from a sharp instrument such as a needle or syringe includes a sheath slidably attached to the mounting. The sheath is locked in an open position during use of the needle and syringe. After use, the sheath is rotated to unlock the protective mechanism. A spring urges the sheath over the needle, thereby enclosing the needle and protecting anyone handling the instrument from injury. The sheath is maintained in its enclosing position by a locking flange in the interior of the sheath which engages an abutting ring on the mounting.