Plug-in connector for hose and/or pipe connections, comprising - a coupling part having an outer periphery and having a receiving opening into which a plug part with a plug shaft, which has a collar, can be inserted in a plug-in direction, and - a radially slotted retaining clip, which is resiliently elastic at least in regions, has at least two latching arms (15a, 15b), can be pre-mounted on the coupling part as a result of radially latching the latching arms (15a, 15b) to the coupling part and can be radially moved between an insertion position for the plug part and a retaining position for the plug part, wherein the plug part, in the insertion position, can be inserted into the receiving opening through the retaining clip, and wherein the plugged-in plug part with its collar in the retaining position can be blocked against being pulled out in the receiving opening in a form-fitting manner by means of the retaining clip, wherein the retaining clip is held locked in the insertion position in such a way that the retaining clip protrudes in relation to the outer periphery of the coupling part by way of a projection, and wherein the retaining clip is held locked in the retaining position in such a way that the retaining clip terminates flush with the outer periphery of the coupling part, wherein the basic shape of the retaining clip is of a U-shaped form as viewed in the direction of the longitudinal axis (X-X), wherein the legs of the U form two pairs (15a/15b, 16a/16b) of spring arms (15a, 15b, 16a, 16b) which are located diametrically opposite one another, wherein the pairs (15a/15b, 16a/16b) of the spring arms (15a, 15b, 16a, 16b) are arranged one behind the other in the axial direction (X-X) and the pairs (15a/15b, 16a/16b) of spring arms (15a, 15b, 16a, 16b) that are located axially (X-X) one behind the other are separated from one another in each case by a slot, and wherein an actuating surface for the retaining clip is formed by the connecting region of the legs of the U on the side that faces away from the spring arms (15a, 15b, 16a, 16b), characterized in that the spring arms (15a, 15b) of only one pair (15a/15b) of spring arms (15a, 15b, 16a, 16b) form latching arms (15a, 15b) for radially latching the plug part, wherein guide slots (24a, 24b) for engaging complementary guide pins (23a, 23b) situated on the spring arms (15a, 15b) of the retaining clip are situated in the coupling part, and specifically in the receiving body thereof for the retaining clip, on an end wall of the coupling part that faces the insertion direction of the plug part, wherein a control contour is formed in each of the guide slots (24a, 24b), by means of which control contour, in interaction with the complementary guide pins (23a, 23b), when the guide pins (23a, 23b) are displaced in their guide slots (24a, 24b) for the purpose of assembly during the insertion of the retaining clip into the coupling part, proceeding from an end stop into the region of a first bend angle with spreading of the spring arms (15a, 15b), on which the guide pins (23a, 23b) are situated, in each case a retaining lug is reached which increases the resistance against further insertion of the retaining clip. - a coupling part having an outer periphery and having a receiving opening into which a plug part with a plug shaft, which has a collar, can be inserted in a plug-in direction, and - a radially slotted retaining clip, which is resiliently elastic at least in regions, has at least two latching arms (15a, 15b), can be pre-mounted on the coupling part as a result of radially latching the latching arms (15a, 15b) to the coupling part and can be radially moved between an insertion position for the plug part and a retaining position for the plug part, wherein the plug part, in the insertion position, can be inserted into the receiving opening through the retaining clip, and wherein the plugged-in plug part with its collar in the retaining position can be blocked against being pulled out in the receiving opening in a form-fitting manner by means of the retaining clip, wherein the retaining clip is held locked in the insertion position in such a way that the retaining clip protrudes in relation to the outer periphery of the coupling part by way of a projection, and wherein the retaining clip is held locked in the retaining position in such a way that the retaining clip terminates flush with the outer periphery of the coupling part, wherein the basic shape of the retaining clip is of a U-shaped form as viewed in the direction of the longitudinal axis (X-X), wherein the legs of the U form two pairs (15a/15b, 16a/16b) of spring arms (15a, 15b, 16a, 16b) which are located diametrically opposite one another, wherein the pairs (15a/15b, 16a/16b) of the spring arms (15a, 15b, 16a, 16b) are arranged one behind the other in the axial direction (X-X) and the pairs (15a/15b, 16a/16b) of spring arms (15a, 15b, 16a, 16b) that are located axially (X-X) one behind the other are separated from one another in each case by a slot, and wherein an actuating surface for the retaining clip is formed by the connecting region of the legs of the U on the side that faces away from the spring arms (15a, 15b, 16a, 16b),