Plug-in connection for fluid lines, comprising two plug-together coupling parts (2, 3) which, in the plugged-in state, can be arrested in a releasable manner via a locking device, which belongs to the plug-in connection and comprises, on the one hand, two outer, radial, flange-like retaining crosspieces (21, 31) of the two coupling parts (2, 3), said retaining crosspieces, in the plugged-in state, being axially (X-X) adjacent in the plug-in direction, and, on the other hand, a retaining part which has an annular contour, is designed in the form of a single-part elastic ring with a circumferential contour interrupted by a slit and, in a preassembly state, can be mounted in captive fashion in axially and radially fixed manner on one of the two coupling parts, and, in the assembled state, engages axially and radially around the retaining crosspieces (21, 31) of the two coupling parts (2, 3) by way of at least one radially elastically movable retaining portion, which is C-shaped in axial section and has two collars (51, 52), wherein one collar in the region of the retaining portion is designed in the form of a latching element which, on the side which is directed towards the coupling part during the plug-in operation, has a radially inner sloping surface, which is designed in the form of a spreading surface and causes a radially outwardly directed spreading movement of the retaining part as a result of the retaining crosspiece of the coupling part moving relative to it during the plugging-in operation, wherein said collar, following latching to the coupling part, engages in a form-fitting manner with latching action behind the retaining crosspiece by way of a radial retaining edge, wherein an elastic, stop-forming actuating region is formed on the circumference between two retaining regions, which are located in the longitudinal sides of the retaining part, on a transverse side of the retaining part which is located diametrically opposite the slit, wherein subjecting a pressure-exerting surface of the actuating region to a compressive force (p), which for dismantling purposes acts in the direction of a longitudinal axis (Y1-Y1) of the retaining part, can give rise to the retaining portions widening radially as a result of a spreading-open operation, wherein the actuating region is configured such that when, after covering an actuating distance (a), it ends up in abutment against one of the coupling parts (2, 3), it delimits the spreading movement of the retaining part, that is to say the radially elastic movement of each retaining portion in the outwards direction, wherein the collars (51, 52) of the retaining part are designed to have different sizes and/or shapes such that, in the case of the retaining portion moving radially elastically outwards out of the assembled state, a first collar, which in the assembled state butts against the retaining crosspiece of the one coupling part, frees said retaining crosspiece of the coupling part, whereas, the other, second collar still engages, at least in parts, behind the retaining crosspiece of the other coupling part, against which it butts, the collars (51, 52) differing in size and/or shape in that, in the assembled state, the one collar encompasses one of the coupling parts (2, 3) over a central angle (μ1) which is larger than a central angle (μ2) over which the other collar encompasses the other coupling part (3, 2).