Citti, J. E. (Oregon State University, Corvallis), W. E. Sandine, and P. R. Elliker. beta-Galactosidase of Streptococcus lactis. J. Bacteriol. 89:937-942. 1965.-Synthesis of beta-galactosidase by several strains of Streptococcus lactis was induced by lactose. The rate of hydrolysis of o-nitrophenyl-beta-d-galactopyranoside was used to measure enzyme activity. The enzyme of all but one strain was unstable when whole cells were sonic-treated or treated with toluene; the enzyme of one strain of S. lactis was stable to these treatments, which resulted in at least a fivefold increase in activity over that found in whole cells. The optimal assay conditions for toluene-treated cells of this strain involved incubation at 37 C in pH 7.0 sodium phosphate buffer. Lactose was the most effective inducer of enzyme synthesis. Methyl-beta-d-thiogalactopyranoside, isopropyl-beta-d-thiogalactopyranoside, and galactose were also inducers of the enzyme, but were not as effective as lactose. Melibiose, maltose, and calcium lactobionate were poor inducers of enzyme synthesis. Exogenously supplied glucose repressed enzyme synthesis. The means of control of induced beta-galactosidase synthesis in S. lactis was similar to that in Escherichia coli.