A β-D-glucosidase (linamarase) was purified 11,700-fold from the butter bean, Phaseolus lunatus L., by means of successive procedures including extraction, ammonium sulfate fractionation, acetone treatment, and chromatographies on CM-Sephadex, DEAE-Sephadex, and Sephadex G-200. The final preparation gave a single protein band on both disc polyacrylamide gel electrophoresis and SDS-polyacrylamide gel electrophoresis. In spite of its electrophoretic purity, the final enzyme preparation showed four glycosidase activities; β-D-glucosidase, β-D-galactosidase, β-D-fucosidase, and β-D-xylosidase. The molecular weight of the enzyme was determined to be 124,000 ±9,000 by Sephadex G-200 gel filtration, and 59,000±2,400 by SDS-disc gel electrophoresis. The enzyme showed a pH optimum in the range of 5.1 to 6.0 with p-nitrophenyl β-D-glucoside, 4-methylumbelliferyl β-D-glucoside, and linamarin. Among natural substrates containing a β-glucosyl terminal, linamarin, prunasin, and salicin were hydrolyzed by the enzyme from butter beans, but amygdalin, cellobiose, gentiobiose, and laminarin were hardly hydrolyzed.