An endo-1,4-beta-D-galactanase (EC 3.2.1.89) was purified to homogeneity from a solid-state culture of Aspergillus sojae. The molecular weight of the galactanase was estimated to be 39,700 by sodium dodecyl sulfate-polyaclylamide gel electrophoresis (SDS-PAGE). Gel filtration chromatography indicated the native enzyme to be a monomer. The isoelectric point of the galactanase was 3.60. The optimum pH and temperature of the enzyme activity were 4.5 and 50 degrees C, respectively. The galactanase was stable from pH 6.0 to 10.0, and up to 35 degrees C. The K-m value for arabinogalactan from soybean was 0.82 mg/ml. The activity of the enzyme was significantly inhibited by Mn2+, Hg2+, Ag+, and Fe3+, and no stimulation by metal ions was apparent. After the hydrolysis of arabinogalactan from soybean, the major products were galactobiose and galactose, and no liberation of arabinose was observed in the reaction mixture.