A tubular fitting for medical fluid lines includes a hollow body within which a male connector and an opposite female connector are coaxially housed. The male connector is coupled in rotation with respect to the body in a direction corresponding to a screwing with a female connector of the line, while it is normally freely rotatable in the unscrewing direction, in the same way as the female connector is coupled in rotation with respect to the body in the direction corresponding to the screwing with a male connector of the line, while it is normally freely rotatable in the unscrewing direction. The male connector and/or the female connector can possibly be locked in rotation with respect to the body in the unscrewing direction, only following a positive command.