A series of plasmid vectors has been constructed for the regulated, high level expression of foreign genes in E. coli. The vectors express cloned genes under the control of the tac promoter, which is a hybrid of trp and lac promoter sequences. Some of our expression vectors carry in addition to the tac promoter, the efficient lacZ ribosome binding site followed by unique cloning sites. These vectors can be used to express cloned genes directly, i.e. in an unfused, mature form. A second type of vector provides, in addition to the above regulatory elements, a translation initiation codon (ATG) for the expression of genes which have been isolated in an incomplete form (for example: cDNA). A third type of vector allow readily the construction of gene fusions to the E. coli beta-galactosidase gene, which may stabilize otherwise unstable eukaryotic proteins, and thus allows the production of high amounts of specific antigens in E. coli. With the above vectors, several eukaryotic and viral proteins, including SV40 small tumor antigen, human fibroblast interferon and herpes simplex glycoproteins have been expressed.