A tissue processor is disclosed for processing biological tissue, for example, for mounting on microscope slides. The tissue processor has a valve arrangement in the bottom of a tissue processing chamber, whereby a selected one of a plurality of fluid ports formed in a base of the tissue processing chamber may be connected with the interior of the chamber for supply of processing fluid thereto or discharge of fluid from the chamber. In this way different processing fluids may be passed into and removed from the processing chamber in succession, for processing of tissue samples contained in baskets within the chamber. The valve arrangement comprises a rotary disc at the bottom of the chamber and in sealing engagement with a valve surface on the base of said chamber, the valve surface being provided with a plurality of ports connected with respective said fluid inlets. The rotary disc is provided with a single port therethrough so that by rotating the disc into a selected position the port in the rotary disc can be brought into register with a selected one of the plurality of fluid ports formed in the base of the tissue processing chamber.