Induction of the tetracycline-resistance genes function by the inducer of the DNA-repair and mutability SOS-system, UV-light, has been tested. Activity of the tet-genes residing on the plasmid RP4 in Escherichia coli cells has been shown to be inducible by the low doses of tetracycline as well as by the mutagenic doses of UV-light. The induction was quantitatively registered by measuring the activity of beta-galactosidase of bacteriophage Mud1 (Ap, Lac) inserted into the tet-genes of the plasmid RP4. The bacteriophage integration inactivates the tet-genes function of the constructed plasmid fusing the lac-operon to a promoter of inactivated genes. Precise excision of bacteriophage restores the activity of the tet-genes proving together with the plasmid DNA-restriction analysis the fusion of tet-promoter with Iac-operon. The tet-genes of RP4 are concluded to be a part of the SOS-regulon, a set of genes inducible by the conditions harming the bacterial cell. Preliminary data on the mutagenic activity of tetracycline obtained in the bacterial test-system of mutagens are discussed.