A system for processing biological specimens mounted on microscope slides by adding and removing processing fluids from microscope slides by means of capillary action using a slide holder capable of holding multiple microscope slides, and a spacer positioned in between two slides of a slide pair to create a capillary gap. A capillary gap adjuster can be used to pinch and release one end of the slide pair to create a pulsatile action to mix the reagent within the capillary gap. The system may further include a reagent holder, an absorbent pad, a series of reagent baths, and an incubator for holding one or more slide holders.