Fava Nettos' polysaccharide antigen (FNPA), shown to detect humoral and cellular responses in paracoccidioidomycosis, was investigated. Skin tests with FNPA were negative after alkaline hydrolysis or depletion of gp43 peptide epitopes. Purified antibodies to FNPA or gp43 from paracoccidioidomycosis patients cross-reacted, showing common epitopes and FNPA-specific ones. Normal human sera, in contrast to paracoccidioidomycosis sera, were unreactive with gp43 but recognized epitopes of FNPA susceptible to alkaline hydrolysis and pronase treatment. Histoplasmosis patients sera strongly reacted with FNPA carbohydrate epitopes.