This solution is highly effective at revealing antigens in formalin-fixed, paraffin-embedded tissue sections when used in combination with a high temperature treatment procedure. Although most antigens are revealed or are more accessible to antibodies using a slightly acidic pH formula, some require a higher pH antigen unmasking solution. Antigen unmasking solutions have now become standard companion reagents for use with most monoclonal antibodies. This Antigen Unmasking Solution is supplied as a concentrated stock of a high pH TRIS-based formula sufficient to prepare over twenty-five liters of working solution. A detailed protocol describing dilution and optimal conditions for use is included.