The fitting (1) comprises as an enbloc piece a first hollow part (1a), suitable for connecting with a tube of a bag, a second hollow part (1b) suitable for engaging with a cannula, a pierceable membrane (4) that separates the two parts, a cap (2) suitable for closing the lower aperture of the fitting and possibly housing a membrane (22) adapted to filter a sterilization gas (G) introduced into the second part (1b) -