Several Saccharomyces cerevisiae strains with a super-secreting phenotype have been transformed using a secretion plasmid containing the LAC4 gene and have proven to be effective in the secretion of Kluyveromyces lactis beta -galactosidase. The strain CGY1585 (ssc1-1) showed the highest secretion (1.7 EU ml(-1)) in the culture medium. As far as we know, Kluyveromyces lactis beta -galactosidase is the largest sized protein and the only intracellular one among those secreted by these mutants hitherto. The recombinant strains all grew in lactose media.