Filing Date: 2003-02-14.-- Priority Data ES P200200419 (2002-02-21)The invention relates to a method for the hydrolysis of lactose catalysed by immobilised/stabilised recombinant derivatives of β-galactosidase from Thermus sp. (strain t2) fused with polyhistidine tails (imac/?-galactosidase) and cloned in E. coli CECT 5970. The inventive method is suitable for use in the dairy industry and in biotechnology where it is applied to different milk derivatives, such as milk, whey or any other compound comprising lactose, in order to prevent intolerance problems and improve the organoleptic properties thereof. With respect to the environment, the method can be used to eliminate highly-contaminating lactose waste (highly-reducing sugar), by transforming said waste into a product with added value (fructose and glucose syrup). The enzyme is thermoresistant and contains no impurities and, moreover, the preparation method thereof involves single-step purification which is carried out with a mesophilic microorganism.