This study was designed to improve current methods for demonstrating fungi in sections of formalin-fixed, paraffin-embedded tissue by the fluorescent antibody technic. The findings in this study indicate that digestion of sections in 1.0% trypsin solution for 1 hr. at 37 C. before conjugates are applied is a single, inexpensive procedure that enhances considerably the staining of Blastomyces dermatitidis, Coccidioides immitis, Cryptococcus neoformans, Histoplasma capsulatum, and Sporotrichum schenckii. The results of this study also demonstrate that these fungi can be stained by the fluorescent antibody technic in tissue sections that have been stained previously by the hematoxylin and eosin, the Brown and Brenn, and the Giemsa procedures. They cannot be stained, however, in sections that have been stained previously by the Gomori methenamine-silver nitrate, the periodic acid-Schiff, or the Gridley procedures. Apparently, the oxidation of polysaccharides in the walls of these fungi by the periodic acid or the chromic acid alters the antigenicity of these organisms so that they no longer react with the labeled antibodies. The addition of dimethyl sulfoxide to conjugates in a ratio of 1:9 or 1:14 was found to improve the fluorescent antibody staining results obtained with these fungi in formalinized tissue sections. This improvement is the result of enhancement of contrast between stained organisms and the background.