The gene encoding the beta-galactosidase from the dairy Lactococcus lactis IL1403 strain was cloned, sequenced and overexpressed in Escherichia coli. The purified enzyme has a tetrameric arrangement composed of four identical 120 kDa subunits. Biochemical characterization showed that it is optimally active within a wide range of temperatures from 15 to 55 A degrees C and of pH from 6.0 to 7.5. For its maximal activity this enzyme requires only 0.8 mM Fe2+ and 1.6 mM Mg2+. Purified protein displayed a high catalytic efficiency of 102 s(-1) mM(-1) for lactose. The enzyme stability was increased by immobilization mainly at low pH (from 4.0 to 5.5) and high temperatures (55 and 60 A degrees C). The bioconversion of lactose using the L. lactis beta-galactosidase allows the production of lactose with a high bioconversion rate (98 %) within a wide range of pH and temperature.