Endoinulinase from Aspergillus ficuum, which catalyzes the hydrolysis of inulin via an endo-cleavage mode, was purified by chromatography from Novozym 230 as a starting commercial enzyme mixture on CM-Sephadex and DEAE-Sepharose, and by preparative electrophoresis under native conditions. The enzyme was estimated to be pure on the basis of its I/S ratio, whose value was infinite in our assay conditions. Two forms separated by using this method. SDS gel electrophoresis showed the two purified forms to respectively exhibit molecular weights of 64,000 +/- 500 and 66,000 +/- 1,000. The results of deglycosylation indicated that the two forms were originally the same protein but with different sugar contents. A molecular weight of 54,800 +/- 1,500 was found by gel filtration of the native enzyme, indicating the native functional protein to be a monomer. The enzyme showed nearly absolute substrate specificity towards inulin and inulooligosaccharides, and acted via an endo-attack to produce mainly inulotriose during the late stage of the reaction. The apparent Km and Vmax values for inulin hydrolysis were 8.1 +/- 1.0 mM and 773 +/- 60 U/mg, respectively. The internal peptides of the enzyme showed sequence homology to the endoinulinase of Penicillium purpurogenum.