A medical device having a releasable retaining includes needle cannula (21) connected to a hub (31) having an open proximal end (32) with a frusto-conically shaped cavity (33) therein. The cavity is part of a passageway (35) through the hub (31) which is in fluid communication with the lumen (24) of the needle cannula (21). A retaining element (41) is releasably connected to the hub (31). The hub (31) includes an aperture (43) and at least one protuberance (44) projecting into the aperture (43) for engaging the frusto-conically shaped tip (57) of a fluid delivery device (51). The protuberance (44) is shaped to offer less resistance to hub (31) movement in a direction of engagement than in a direction of disengagement with the tip (57). The retaining element (41) is connected to the hub (31) so that when the hub (31) is engaged to the tip (57), the force required to disengage the retaining element (41) from the tip (57) is greater than the force required to disengage the retaining element (41) from the hub (31).
