Enzymatic and non-enzymatic treatments for antigen unmasking on formalin-fixed, paraffin-embedded, dewaxed sections were optimized and compared by the use of a panel of antibodies of diagnostic relevance (anti-cytokeratins, vimentin, S-100, T- and B-cell receptors, Ki-67/MIB 1, muscle actin). Non-enzymatic unmasking was obtained by boiling the slides in a microwave oven in 0.01 M salt solution (pH 6) or in 6 M urea. Trypsin or pronase digestion was used for comparison and found to be necessary for some of the reagents. The investigation was then extended to 256 antibodies; the epitopic amino acid sequence was known for 48 of them. We found that enzymatic and non-enzymatic antigen unmasking are not dependent on the epitope sequence, but some antigens benefit selectively from one treatment but not from the other. Denaturation of proteins is the likely mechanism which leads to immunodetection on microwave oven-boiled slides; this suggestion is supported by the use of denaturating solutions and by the observation that endogenous enzymes were inactivated and a few antigens were no longer immunodetectable after boiling. Non-enzymatic methods for antigen unmasking are a powerful new tool for broadening the use of antibodies for immunostaining formalin-fixed, paraffin-embedded sections and should be used in parallel with the traditional enzymatic methods.