Bifidobacterium breve 203 grew well on soymilk, reaching a cell number of 2×109 cells/ml on 24 hr cultivation, at which time the medium had solidified and its pH had decreased to 4.0. Stachyose and raffinose in the medium were assimilated in preference to sucrose. A crude extract of B. breve 203 hydrolyzed the oligosaccharides in soymilk almost completely into their constituent monosaccharides. The α-D-galactosidase responsible for the first reaction in the degradation of the α-D-galactosyl oligosaccharides was isolated from a crude extract of the organism (500-fold purification, 2% yield) by means of ammonium sulfate-fractionation and column chromatographies on DEAE-cellulose, DEAE-cellulofine, Sepharose 6B, hydroxylapatite, Sephacryl S-300 and Phenyl-Sepharose 6B. The enzyme was a homo-octamer with a molecular weight of about 330, 000, and its isoelectric point was 3.7. It reacted optimally at pH 5.5 and hydrolyzed Stachyose (Km 8.6 mM), raffinose (Km 4.4 mM) and melibiose (Km 3.0 mM), with 50-60% of its reactivity toward p-nitrophenyl α-D-galactoside (Km 0.16mM). The reaction of the purified α-D-galactosidase with soymilk and guar gum was also investigated.