To study the structures of the epitopes of the extracellular polysaccharides from Penicillium and Aspergillus species, an exo-beta-D-galactofuranosidase was purified from a commercial crude enzyme preparation from Trichoderma harzianum. Analysis of ring size and linkage position of the galactose residues of the extracellular polysaccharide of Penicillium digitatum, before and after enzymatic treatment, was determined by the reductive-cleavage technique. In addition to terminal and beta (1-5)-linked galactofuranosides, beta (1-6)-linked and beta (1,5,6)-linked branched galactofuranose residues could be identified. After degradation with the purified exo-beta-D-galactofuranosidase, all initial linkages of the galactofuranose residues were still present, but the amount of beta (1-5)-linked galactofuranose residues had decreased considerably. Treatment of the extracellular polysaccharides of Penicillium and Aspergillus species with the purified exo-beta-D-galactofuranosidase resulted in complete disappearance of the enzyme-linked immunosorbent assay reactivity of these polysaccharides, using immunoglobulin G antibodies raised against P. digitatum. Therefore, with the use of this enzyme, it was proved that the beta (1-5)-linked galactofuranosyl residues only are responsible for the antigenicity of the extracellular polysaccharides of Penicillium and Aspergillus molds. A new structural model for the antigenic galactofuranose side chains of the galactomannan from P. digitatum is proposed.