The structural changes of the tet operator DNA upon binding of the TET repressor protein are examined by circular dichroism. For this purpose a 70 bp DNA fragment was prepared which contains both tet operators. About 67% o'f the base pairs of this DNA are involved in specific interaction with the TET repressor.A rather large change in the CD of the DNA is induced by binding of the TET repressor. The shape of the CD difference spectrum is similar to the respective difference found for the lac operator DNA upon complex formation with the lac repressor. However, the effect induced by the TET repressor on tet operator DNA seems to comprise both the specific and non-specific effect of the lac repressor on the structure of DNA [Culard, F. and Maurizot, J.C. (1981) Nucl. Acids Res. 9, 5157-5184]. Specificity of binding is confirmed by the lack of any effect of the TET repressor on the CD of a 95 bp lac operator containing DNA fragment, by the reduced mobility of TET repressor-tet operator complexes on polyacrylamid gels under CD conditiones, and by a titration experiment of tet operator DNA with TET repressor employing the CD change. The latter experiment reveals a stoichiometry of four TET repressors per tet operon control region.