β-Galactosidase fromStreptococcus lactis7962 was partially purified and enzymatic and chemical properties were determined. The enzyme was unstable in phosphate buffer at 2 °C, dissociating into two forms and losing activity at the rate of 1.0% per hour. Dissociation was prevented and activity stabilized by treatment of enzyme extracts with 0.85 Mammonium sulfate. The two forms of the enzyme were separable by gel filtration on Sephadex G-200 equilibrated with buffer containing 0.85 Mammonium sulfate. These enzyme forms were indistinguishable on the basis ofKm(10−3 M), temperature (37.5 °C), and pH (7.05) optima. From molecular weight determinations of the two forms, it appeared that the form (dimer) having the larger molecular weight was composed of two monomers. Holding enzyme extracts at 27 °C for 22 h caused the dimer enzyme form to increase in amount while the monomer correspondingly decreased; ammonium sulfate stabilized cell extracts contained only the dimer form, suggesting that dissociation and reaggregation occurred in non-salt stabilized preparations. Culture age at the time of enzyme extraction also influenced the relative amounts of the two enzyme forms present; the dimer form decreased and monomer increased as the cells passed through the rapid growth phase; both forms of the enzyme decreased as growth slowed down.