An α-d-galactosidase (EC 220.127.116.11), capable of converting group B into group O red cells, was isolated from the stem portion of taro. It was purified about 3000 fold by gel filtration and ion-exchange chromatography. The blood group-converting activity was demonstrated by hemolysis and hemagglutination studies. This activity is comparable to that of α-d-galactosidase isolated from coffee beans. Taro α-d-galactosidase also hydrolyzes (1→4)- and (1→6)-linked α-d-galactopyranosyl groups from d-galactose-containing glycoconjugates. Taro α-d-galactosidase has a low Km value (0.28mM), a low molecular weight (40 000), and a neutral optimal pH (6.0). At a final enzyme concentration of 30 units/mL in the incubation mixture, the conversion of group B into group O activity was completed within two hours, without apparent changes in the shape of the red cells.