Kluyveromyces lactis beta-galactosidase gene, LAC4, was expressed in Escherichia coli as a soluble His-tagged recombinant enzyme under the optimized culture conditions. The expressed protein was multimeric with a subunit molecular mass of 118 kDa. The dimeric form of the beta-galactosidase was the major fraction but had a lower activity than those of the multimeric forms. The purified enzyme required Mn2+ for activity and was inactivated irreversibly by imidazole above 50 mM. The activity was optimal at 37 and 40 degreesC for omicron-nitrophenyl-beta-D-galactopyranoside (omicronNPG) and lactose, respectively. The optimum pH value is 7. The K-m and V-max values of the purified enzyme for omicronNPG were 1.5 mM and 560 mumol min(-1) mg(-1), and for lactose 20 mM and 570 mumol min(-1) mg(-1), respectively.