The enzyme β-amylase (1,4-α-d-Glucan maltohydrolase, E.C. 3.2.1.2) was isolated and purified to homogenity from the extract of healthy radish roots (Raphanus sativus). The purification involved ammonium sulphate precipitation (100% saturation); DEAE-cellulose; hydroxylapatite and Sephadex G-200 chromatography. The purity and homogenity of the enzyme preparation were judged by gel filtration on Sephadex G-200 and disc electrophoresis. The amount of the original enzyme activity remaining was 23% after 195·2 times purification, with specific activity 820 U/mg protein. The enzyme was active against starch, glycogen and α-dextrin but it failed to hydrolyze sucrose, maltose and lactose. Its molecular weight was 58 880 daltons, as estimated by gel filtration on Sephadex G-200. The K_m value was 2·85% for soluble starch at optimum pH 5·0 and 45°C. The enzyme was relatively heat-stable for 15 min at 30 and 40°C, showing only about 8% loss of activity. The enzyme was completely inactivated by Cu^{+ 2}, Fe^{+ 2}, Ag⁺, Hg^{+ 2}, but only moderately inhibited by p-chloromercuribenzoate. Strongly activated enzyme was obtained with EDTA, Zn^{+ 2}, K⁺, Ca^{+ 2} and Co^{+ 2}.