Plug-in connector for connecting a first fluid line to a mating plug-in connector, comprising a housing having a passage duct, wherein one end of the housing is in the form of a ferrule portion with a receiving duct, which is fluidically connected to the passage duct, for receiving an adapter sleeve belonging to the plug-in connector, wherein the adapter sleeve can be inserted into the receiving duct of the ferrule portion in a fitting direction directed axially to a fitting axis and is releaseably held in the receiving duct in a form-fitting manner in the axial direction, wherein the adapter sleeve has formed a passage opening for a plug shank of the mating plug-in connector and has a holding means for releaseably securing the mating plug-in connector, wherein the holding means, in an inoperative state, protrudes into the passage opening and is designed to elastically expand into a tensioned state radially with respect to the fitting axis, and the plug shank of the mating plug-in connector can be inserted into the passage opening of the adapter sleeve in a pre-fitting position of the plug-in connector and the holding means can block the mating plug-in connector axially with respect to the fitting axis in a fitting position of the plug-in connector, wherein the adapter sleeve has two windows which are situated opposite each other at an 180° angle and are radially open with respect to the fitting axis, and a locking element which is movable axially with respect to the fitting axis is arranged on an outer circumference of the ferrule portion in a manner movable from a release position, in which the holding means is released, to a locking position, in which the holding means is locked, wherein the locking element, in its release position in the pre-fitting position of the plug-in connector on the ferrule portion, is secured at least axially against movement to the locking position, wherein a reinforcing strut extending axially with respect to the fitting axis is arranged in each of the windows, wherein the holding means has two holding arms and by way of the holding arms passes through the windows in such a way that, in the inoperative state, the holding arms of the holding means are arranged radially between the passage opening and the reinforcing struts, so that the holding means, by way of the holding arms, protrudes into the passage opening of the adapter sleeve at least in the pre-fitting position. wherein the adapter sleeve can be inserted into the receiving duct of the ferrule portion in a fitting direction directed axially to a fitting axis and is releaseably held in the receiving duct in a form-fitting manner in the axial direction, wherein the adapter sleeve has formed a passage opening for a plug shank of the mating plug-in connector and has a holding means for releaseably securing the mating plug-in connector, wherein the holding means, in an inoperative state, protrudes into the passage opening and is designed to elastically expand into a tensioned state radially with respect to the fitting axis, and the plug shank of the mating plug-in connector can be inserted into the passage opening of the adapter sleeve in a pre-fitting position of the plug-in connector and the holding means can block the mating plug-in connector axially with respect to the fitting axis in a fitting position of the plug-in connector, wherein the adapter sleeve has two windows which are situated opposite each other at an 180° angle and are radially open with respect to the fitting axis, and a locking element which is movable axially with respect to the fitting axis is arranged on an outer circumference of the ferrule portion in a manner movable from a release position, in which the holding means is released, to a locking position, in which the holding means is locked, wherein the locking element, in its release position in the pre-fitting position of the plug-in connector on the ferrule portion, is secured at least axially against movement to the locking position, wherein a reinforcing strut extending axially with respect to the fitting axis is arranged in each of the windows, wherein the holding means has two holding arms and by way of the holding arms passes through the windows in such a way that, in the inoperative state, the holding arms of the holding means are arranged radially between the passage opening and the reinforcing struts, so that the holding means, by way of the holding arms, protrudes into the passage opening of the adapter sleeve at least in the pre-fitting position.