An improvement in a catheter assembly including a cannula, an inserter having a guide channel therethrough in which the cannula slidably fits and a hub structure with the cannula proximal end portion attached to the hub structure. The improvement includes a sheath having a longitudinal slit or weakened portion and being about the cannula between the inserter and the hub structure. A sheath stripping construction carried by the inserter strips the sheath from about the cannula as the cannula slides distally through the guide channel. A first lock member is carried by the inserter and an interlocking second lock member is located about the cannula between the inserter and the hub structure. The cannula is protected from contamination and from kinking and a positive lock serves to prevent its accidental withdrawal. A method of inserting a cannula using such an assembly is likewise set forth.