Arrangement comprising a retaining spring for fixing a mounting body of a luminaire to a mounting rail of the luminaire, wherein the mounting body and the mounting rail are each designed elongate in a longitudinal direction and the arrangement comprises the mounting body and the mounting rail, wherein the mounting body has two mounting body sidewalls spaced from each other in a transverse direction perpendicular to the longitudinal direction and connected to each other via a mounting body base, on which mounting body sidewalls a first mounting body fixing anchor is respectively provided which is designed in particular as a mounting body projection, and wherein the mounting rail has two mounting rail sidewalls spaced from each other in the transverse direction, on which mounting rail sidewalls a mounting rail fixing anchor is respectively provided which is designed in particular as a mounting rail projection, wherein the mounting rail has a cross section perpendicular to the longitudinal which is open at one vertical end along a vertical direction perpendicular to the longitudinal direction and to the transverse direction and which is bounded along the transverse direction by two mounting rail sidewalls which are connected to each other by a mounting rail base extending along the transverse direction and which extend vertically away from the mounting rail base, wherein the retaining spring has two retaining sections opposite each other in the transverse direction, wherein in the fixing position of the arrangement, in which the mounting body is disposed at the open vertical end of the mounting rail so that the mounting body together with the mounting rail forms an interior space that is circumferentially closed perpendicular to the longitudinal direction by the mounting rail and the mounting body, the mounting rail is fixed relative to the mounting body by the retaining spring in that the retaining spring presses with a base portion of each holding section vertically downwards against a mounting body section that is formed in particular by the mounting body base, presses with a first retaining portion of each holding section vertically upwards against one of the mounting body fixing anchors and presses with a second holding portion of each holding section vertically downwards against a mounting rail fixing anchor, characterized in that each of the holding sections is formed by a continuous wire, wherein in particular the wire which forms the respective retaining section has a wire length of at least 100 mm and in particular each of the retaining sections has a longitudinal extent of at least 40 mm in a longitudinal direction perpendicular to the transverse direction and perpendicular to the vertical and has a vertical extent of at least 15 mm, wherein at least one of the retaining sections has an actuating portion (5, 150) and the actuating portion (5, 150), the base portion (6, 160) and the retaining portions of the retaining section are designed to correspond to each other in such a way that the wire (2, 102) forming the retaining section is elastically twisted and/or elastically bent when the actuating portion (5, 150) is deflected along the transverse direction from the fixing position towards the opposite retaining section, and are designed to correspond to each other in such a way that the second retaining portion of the retaining section can be released from the mounting rail fixing anchor by actuating the actuating portion along the transverse direction, starting from the fixing position, so that the mounting rail fixing anchor and the second holding portion do no longer overlap in the transverse direction, while the first retaining portion continues to overlap with the mounting body fixing anchor in the transverse direction and presses against the latter from below.