A ribonuclease, with a molecular mass of 22.5 kDa and an N-terminal sequence exhibiting resemblance to previously isolated buckwheat storage proteins and allergens, was isolated from Japanese large brown buckwheat seeds. The ribonuclease was purified using a simple protocol that comprised ion exchange chromatography on Q-Sepharose and DEAE-cellulose and gel filtration on Superdex 75. The ribonuclease exhibited low activity toward poly U, lower activity toward poly C, and very low activity toward poly A and poly G. The enzyme was activated upon exposure to 10 mM of Fe(2+) and Zn(2+) ions but was inhibited by Ca(2+), Mg(2+), and Mn(2+) ions at the same concentration. The optimum pH and optimum temperature for the enzyme were pH 9 and 60 °C, respectively. It inhibited proliferation of HepG2 hepatoma and MCF 7 breast cancer cells, with an IC50 value of 79.2 and 63.8 μM, respectively. It potently inhibited HIV-1 reverse transcriptase activity with an IC50 of 48 μM. However, there were no antifungal and mitogenic activities.