1. The p-nitrophenyl β-D-galactosidase asctivity in rat liver homogenates of lysosome-rich fractions was shown to be markedly affected by the ionic composition of the medium. A stimulation of the reaction rate at pH 5 was produced by most of the salts tested, which contained anions such as acetate, SO42− and Cl−, and cations such as Na+, K+ and Mg2+. The most pronounced effect was observed with MgCl2. Only potassium glutamate was inhibitory. 2. Five peaks of β-galactosidase activity obtained by DEAE-cellulose chromatography were equally sensitive to changes in the ionic composition of the medium. In the presence of added NaC1, the whole rate-pH curve was displaced towards higher pH values, the optimum being shifted from 2.0-2.5 to 3.5. The stimulation at pH 5.0 appeared to be mainly due to changes in Vmax., whereas the apparent Km was slightly modified. 3. Unlike the total, the free β-galactosidase activity remained unchanged or even declined when KC1 was added to the reaction medium.