Bifidobacteria assimilated raffinose about 4-fold more effectively than other intestinal bacteria, and alpha-galactosidase was active in all strains of Bifidobacteria tested. The enzyme activity of Bifidobacterium breve grown on raffinose was highly and specifically increased. Its activity was 30-fold higher than that of B. breve grown on glucose. Melibiose was also effective for production of the enzyme. The enzyme was purified to homogeneity from B. breve. It is a homodimer with Mr of about 160 kDa, and its optimum pH for activity of 5.5-6.5. The enzyme showed strict substrate specificity for alpha-galactoside although it had slight activity for alpha-glucoside. It hydrolysed stachyose, melibiose (Km = 2 mM) and raffinose (Km = 0.7 mM).