Microwells, which are open on both sides and are arranged two-dimensionally in a microwell matrix (4), are hermetically closed by pressure on both sides using plugs (6) covered with a sealing sheet (11). A layer of elastic material (9), which is arranged between the plug (6) and the sealing sheet (11), guarantees uniform pressure on all the microwells of a microwell matrix (4) which is loosely held by a frame structure (1). Continuous peripheral elevations (2), which protrude in both directions at right angles to the plane of the microwell matrix (4), ensure that the deformable thin layer of elastic material (9) does not escape sideways between the plug (6) and the microwell matrix (4) during the pressing process. A plurality of microwell matrices (4) are held simultaneously by the frame structure (1) and are hermetically closed by a corresponding number of allocated plugs (6), which are fastened to a base plate (7) and a cover plate (8), respectively.