$eta$-Galactosidase of Bifidobacterium longum KCTC 3215 was studied on the production, purification, and characterization. Optimum conditions for the enzyme production were in the medium of 1.0% lactose as carbon source, initial pH 7.0 and in 17 hours of cultivation at $37^{circ}C$. The enzyme was purified 9.25 folds by protamine sulfate precipitation, ammonium sulfate fractionation, DEAE-Sephadex A-50 ion exchange chromatography and Sephadex G-150 gel filtration. The maximal P-galactosidase activity was observed at pH 6.5 and at the temperature of $40^{circ}C$ This enzyme was stable at pH 6.0-8.5. Metal ions such as $Ca^{2+} ;and ; Co^{2+}$, 2-mercaptoethanol, cysteine, and glutathione stimulated B-galactosidase activity. The enzyme activity was inhibited by addition of $Mg^{2+}, Fe^{2+}, Cs^{1+}, Li^{1+}$, DETA, galactose, and $ ho$-chloromercuribenzoic acid. The kinetics of o-nitrophenyl-$eta$-D-galactopyranoside and lactose were $K_m$ = 1.66 mM, $V_{max}= 0.30 mM/mincdot mgcdot protein$ and $KK_m = 3.18 mM, ; V_{max}= 0.42 mM/min cdot mgcdot$ protein, respectively. The molecular weight of native enzyme was about 360, 000 dalton and the enzyme consisted of 2 identical subunits with a molecular weight of 180, 000.