Abstract: Membrane module for testing active substances in cells with an interior chamber (2) which is limited by a lid (3), a bottom (4), and a side wall (5) and a system of first capillary membranes (6) and a system of second capillary membranes (11) located therein and possibly additional systems of capillary membranes, whereby the capillary membranes in interior chamber (2) are arranged in at least one flat layer parallel to bottom (4) and in interior chamber (2) in the extracapillary space a cell culture chamber is formed around the capillary membranes, whereby the capillary membranes have a lumen that can be filled with a fluid, are passing with at least one of its ends through the side wall (5) of interior chamber (2) and are embedded at this end separated by systems in a casting material (9, 10) so that interior chamber (2) is sealed fluid-tight from the outside, and whereby the capillary membranes of each system are in a fluid connection with their lumens with an inlet device (7) and/or an outlet device (8), a