A medical connector (18) has a visual signal (67) formed by a colored screen-applied band arranged on a bolt (63). In a correct engaged position of the bolt (63) with an external element, the signal is visible. In an incorrect engaged position, the part of the bolt (63) bearing the signal is lowered, so that the signal is hidden from view, providing an indication of a faulty situation. The medical connector serves to connect the end of a dialysate supply tube to an inlet port of a dialyser.