The invention relates to positioning a cannula in a vein. A tab (11) is provided on a movable member (9, 10) comprising a wall (10) with a through-hole (12) therein for inserting a needle in such a way that inserting the needle into the through-hole holds the tab in a retaining position on a flange of the base (2) of a catheter (1), wherein said wall (10) is transversely slidably mounted in the chamber of the needle stick guard (6) relative to the needle between a lower retaining position and an upper release position so that when the needle is retracted into the guard and spaced back from said hole, the wall (10) that no longer has the needle extending therethrough can slide into the upper position thereof, thereby raising said tab above the flange of the base and enabling the guard to be separated from the catheter base. The invention is suitable for use in intravenous catheters.