A covering for apertures of reaction receptacles formed in a microtitration plate, including a hermetic, elastically deforming mat or foil, a frame, and connecting elements. The mat or foil, upon planar deposition on the microtitration plate and following compression, seals the apertures. The connecting elements are fitted onto at least one of the mat or foil and frame and detachably secure the mat or foil to the frame while creating a tension in the mat or foil implementing the connection. The frame (11, 110) is dimensioned so as to permit the frame to be deposited on the microtitration plate (13), such that the mat (12, 120) or foil rests in a planar manner on the microtitration plate, and such that the frame is accessible from above to a pressure and/or heat applying device.