The isolation and properties of a hybrid plasmid carrying the Y gene of the lac operon of Escherichia coli are described. The lactose carrier protein, coded for by the Y gene, is readily identified upon lac operon induction in strains carrying the plasmid. The protein comprises about 15% of the cytoplasmic membrane protein synthesized in the first generation after induction, compared with a wild type strain induced under the same conditions where lactose carrier protein comprises 1.4% of the cytoplasmic membrane protein.