A medical connector, comprising: a first connector member defining a lumen for fluid flow through the first connector member, and comprising a first luer member and a first annular member disposed about the first luer member; a second connector member defining a lumen for fluid flow through the second connector member, and comprising a second luer member and a second annular member disposed about the second luer member; and a check valve disposed in the lumen (1777', 1778') of one of the first and second connector members (1774', 1776') for limiting fluid flow to one di-rection through the medical connector, wherein the first and second annular members (1780', 1784') are adapted to operably engage to securely and releasably connect the first and second connector members (1774', 1776'), and wherein engagement between the first and second luer members (1782', 1786') provides fluid communication be- tween the first and second connector members (1774', 1776'), characterizing in that the check valve comprising: an axially deformable stopper element disposed in the first lumen or the second lumen; and a retaining sleeve disposed in a respective one of the first lumen or the second lumen into which the axially deformable stopper element is disposed, the retaining sleeve defining a central bore and a distal end against which the stopper element is adapted to seat to prevent fluid flow through the respective lumen (1777', 1778') until sufficient pressure is present within the lumen (1777', 1778') to axially deform the stopper element in a longitudinal direction and thereby unseat the stopper element from the distal end of the retaining sleeve, and wherein a receiving cavity accommodating the stopper element is defined by the distal end on the retaining sleeve and a septum and is bounded circumferentially or perimetrically by the lumen (1777', 1778'), and wherein one or more grooves are defined in the wall of the receiving cavity and are adapted to be placed in fluid connection with the central bore of the retaining sleeve for fluid flow through the cavity when sufficient fluid pressure is present within the lumen (1777', 1778') to unseat the stopper element from the distal end of the retaining sleeve. a first connector member defining a lumen for fluid flow through the first connector member, and comprising a first luer member and a first annular member disposed about the first luer member; a second connector member defining a lumen for fluid flow through the second connector member, and comprising a second luer member and a second annular member disposed about the second luer member; and a check valve disposed in the lumen (1777', 1778') of one of the first and second connector members (1774', 1776') for limiting fluid flow to one di-rection through the medical connector, wherein the first and second annular members (1780', 1784') are adapted to operably engage to securely and releasably connect the first and second connector members (1774', 1776'), and wherein engagement between the first and second luer members (1782', 1786') provides fluid communication be- tween the first and second connector members (1774', 1776'), characterizing in that the check valve comprising: an axially deformable stopper element disposed in the first lumen or the second lumen; and a retaining sleeve disposed in a respective one of the first lumen or the second lumen into which the axially deformable stopper element is disposed, the retaining sleeve defining a central bore and a distal end against which the stopper element is adapted to seat to prevent fluid flow through the respective lumen (1777', 1778') until sufficient pressure is present within the lumen (1777', 1778') to axially deform the stopper element in a longitudinal direction and thereby unseat the stopper element from the distal end of the retaining sleeve, and wherein a receiving cavity accommodating the stopper element is defined by the distal end on the retaining sleeve and a septum and is bounded circumferentially or perimetrically by the lumen (1777', 1778'), and wherein one or more grooves are defined in the wall of the receiving cavity and are adapted to be placed in fluid connection with the central bore of the retaining sleeve for fluid flow through the cavity when sufficient fluid pressure is present within the lumen (1777', 1778') to unseat the stopper element from the distal end of the retaining sleeve. an axially deformable stopper element disposed in the first lumen or the second lumen; and a retaining sleeve disposed in a respective one of the first lumen or the second lumen into which the axially deformable stopper element is disposed, the retaining sleeve defining a central bore and a distal end against which the stopper element is adapted to seat to prevent fluid flow through the respective lumen (1777', 1778') until sufficient pressure is present within the lumen (1777', 1778') to axially deform the stopper element in a longitudinal direction and thereby unseat the stopper element from the distal end of the retaining sleeve, and wherein a receiving cavity accommodating the stopper element is defined by the distal end on the retaining sleeve and a septum and is bounded circumferentially or perimetrically by the lumen (1777', 1778'), and wherein one or more grooves are defined in the wall of the receiving cavity and are adapted to be placed in fluid connection with the central bore of the retaining sleeve for fluid flow through the cavity when sufficient fluid pressure is present within the lumen (1777', 1778') to unseat the stopper element from the distal end of the retaining sleeve.