Abstract: The invention relates to a reaction chamber, which includes a cylindrical vessel, with at one end, at least one inlet or outlet tube or lead-through fitted with a valve for a liquid or gaseous component. The other end of the vessel is open. The reaction chamber further comprises a plunger, which performs a reciprocating movement within the cylindrical vessel in axial direction through the open end of the vessel. The plunger tightens against the inner wall of the cylindrical vessel, and has at least one channel, in substantially axial direction, fitted with a valve or connected to a tube fitted with a valve. The invention relates also to an assay method based on the use of the reaction chamber.