A new extracellular ribonuclease (RNase) from a mutant of Aspergillus niger, named A. niger SA-13-20 RNase, was purified to homogeneity by (NH4)(2)SO4 fractionation (50-85%), DEAE-cellulose anion-exchange chromatography, ultrafiltration and Sephacryl HR-200 chromatography. The enzyme was purified up to 54.4-fold with a final yield of 24.5%. There were differences in the molecular weight, pI value and some physico-chemical properties between A. niger SA-13-20 RNase and that from the parent strain. The enzyme is monomeric and its molecular weight and isoelectric point were 40.1 kDa and 5.3, respectively. The N-terminal amino acid sequence of A. niger SA-13-20 RNase was TIDTYSSDSP. The optimum pH, temperature and buffer concentration for the enzymatic reaction were 3.5, 65 degrees C, and 0.175 M, respectively. Metal ions, such as K+, NH4+ Mg2+, and Ca2+ at the concentration of 1.0 mM had a slight activation effect on the enzyme activity and (NH4)(2)SO4 activated the enzyme significantly. The enzyme was stable at pH lower than 8.5 and was easy to inactivate in strong alkali solution.