beta-Galactasidase was extracted from germinating seeds of the black bean, Kestingeilla geocarpa. The enzyme was purified 31-fold by DEAE-cellulose ion-exchange chromatography and Sephadex G-100 gel-permeation chromatography. Galactose, lactose and glucose were competitive inhibitors with K-i values of 26.00, 59.5 and 49 mM, respectively. The inhibition pattern for glucose changed from competitive at low concentrations to uncompetitive at high concentrations. Biphasic thermoinactivation curves were obtained. Galactose protected the enzyme against irreversible thermoinactivation. (C) 2002 Elsevier Science Ltd. All rights reserved.