Abstract: A locking mechanism 10 for a medical sharp device such as a syringe assembly 62 includes a retainer part 12 for retaining a hypodermic needle 22, and a connector part 14. The retainer part includes lugs 46 for engagement with a recess 82 formed in a neck 70 of the syringe assembly. When a plunger 72 of the syringe assembly is pushed fully forwards, the lugs 56 of the retainer part 12 engage behind a ledge 54 of the connector part 14 and the lugs 46 disengage from the recess 82. The plunger 72 may then be retracted, pulling the needle 22 into the barrel 64 of the assembly. The neck portion may be eccentrically mounted on the barrel and means 116,118 may be provided for preventing rotation of the plunger 72 in the barrel 64.