The purpose of this study was to explore a method by which an improved immunofluorescent staining can be applied to formalin-fixed paraffin sections to demonstrate cellular or tissue deposits of immunoglobulins, complement and fibrin, and to demonstrate alpha-1-antitrypsin storage and hepatitis B antigens in liver, toxoplasma in heart, and carcinoembryonic antigens in colonic cancer. It was shown that immunohistochemical demonstration for the above mentioned antigens, but not for complement, was feasible. The paraffin sections were first treated with trypsin and the indirect staining method was used. The trypsin treatment was found to decrease the nonspecific background fluorescence through digestion of the tissue. It probably also unmasked the immunoreactive sites of viral antigens and alpha-1-antitrypsin. In general, a 2-hour digestion was satisfactory for the types of tissues examined in this study, and an optimal period of digestion could be sought to obtain the best result for a specific antigen. This method may be a useful adjuvant to histopathologic study, in which a retrospective immunohistochemical examination may be desirable.