The regulatory region of the tetracycline resistance determinant from transposon Tn10 has been used to construct plasmid vectors for gene expression regulated by tetracycline. Plasmids pRS tetBam-8 and pRS tetBam-16 include the tet regulatory region, the segment coding for the first four amino acids of the tetracycline resistance protein (tetA protein), and a linker region with SalI, HpaII, and BamHI restriction sites for gene fusions. Plasmid pTB-1, a derivative of pRS tetBam-8 and of the beta-galactosidase gene-containing plasmid pMC1403, constitutively expresses a tetA fragment-beta-galactosidase fusion protein. If a multicopy runaway replication plasmid, pMOBglII-16 that includes a 2.7-kb BglII DNA fragment from Tnl10 that provides tetR protein is present along with pTB-1, the expression of beta-galactosidase is reduced eightfold. Tetracycline acts as an inducer of the system and restores the level of beta-galactosidase activity measured in transformants containing pTB-1 alone. Plasmid mutants unable to produce active tetR protein are ineffective in reducing expression. Escherichia coli carrying plasmids that express both tetA protein and tetR protein show an increase in the tetracycline resistance level after incubation with the drug. The observations are consistent with the previously proposed mechanism of regulation of tetracycline resistance in Tn10.