alpha-Galactosidases from mycelial extract and culture filtrate of Aspergillus nidulans have been purified to homogeneity and utilised to obtain polyclonal antibodies anti-alpha-galactosidase. The enzymatic characteristics and the cross reactivity of the antibodies suggest that alpha-galactosidases isolated from the two sources were the same enzyme. Thus, A. nidulans synthesized and secreted only one enzymatic form of alpha-galactosidase which is a multimeric enzyme of 370 kDa composed of four monomers of 87 kDa and a pI of 6.3. The optimum temperature of activity was 50 degrees C and the optimum pH 4-5. The enzyme was stable over a wide range of pH but quite unstable to temperature. alpha-Galactosidase of A. nidulans is a very specific enzyme, it is active only on p-nitrophenyl-alpha-D-galactoside (PNPG), melibiose and raffinose. When PNPG was utilised as substrate melibiose, raffinose, galactose and glucose were competitive inhibitors of the activity.