The expression of the lacZ gene of E. coli in S. cerevisiae has been studied. The enzymatic activity coded by the lacZ gene in E. coli, β-galactosidase, is detectable in yeast cells harboring a chimeric plasmid carrying the gene. On the basis of size and immunological criteria, no difference was detected between the Coli-in-yeast β-galactosidase and the E. coli enzyme.