Hydrolysis of terminal non-reducing beta-D-galactose residues in beta-D-galactosides. Binds 2 magnesium ions per monomer. Can also use manganese. Binds 1 sodium ion per monomer. Inhibited by phenylethyl thio-beta-D-galactoside (PETG), isopropyl thio-beta-D-galactoside (IPTG), L-ribose, D-galactonolactone, lactose and 2-amino-D-galactose. The values for the enzymatic assays using manganese as cofactor are very close. Homotetramer. By allolactose. Belongs to the glycosyl hydrolase 2 family.