We have investigated the use of the Escherichia coli lac operator-repressor system to regulate expression of transfected genes in mammalian cells. We show that lac repressor produced in mouse L cells by transfection of a lacl expression vector blocks transcription of an MSV-CAT fusion gene when the lac operator is inserted at any one of the following sites within the promoter region: between the initiation codon (ATG) and the transcription start site; between the transcription start and TATA box regions; or upstream of the TATA box region. This last result suggests that the repressor may prevent protein-protein interactions involved in transcription activation. The inducer IPTG causes a marked derepression of CAT expression. The lac repressor-operator complex may be useful as an on/off "switch" in the regulation of gene expression for gene transfer experiments.