A recombinant strain of Saccharomyces cerevisiae, secreting beta-galactosidase from Kluyveromyces lactis, grew efficiently with more than 60 g lactose l(-1). The growth rate (0.23 h(-1)) in a cheese-whey medium was close to the highest reported hitherto for other recombinant S. cerevisiae strains that express intracellular beta-galactosidase and lactose-permease genes. The conditions for growth and beta-galactosidase secretion in this medium were optimized in a series of factorial experiments. Best results were obtained at 23 degreesC for 72 h. Since the recombinant strain produced less than 3% ethanol from the lactose, it was also assayed for the production of fructose 1,6-bisphosphate from cheese whey, and 0.06 g l(-1) h(-1) were obtained.