The present invention concerns an assembly for inserting a catheter into a body lumen, the assembly comprising a introduction, an elongate fixing element, wherein with its distal end the fixing element is securable to the introduction device, and wherein at the proximal end of the fixing element a clamping device is provided for securing a guide wire in relation to the introduction device. Further, the assembly comprises a catheter comprising means for releasably engaging the clamping unit, wherein by engagement of the means of the catheter with the clamping unit the guide wire is movable through the introduction device and wherein, upon disengagement of the means of the catheter from the clamping unit, the guide wire is non-movably secured relative to the introduction device.