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Interaction of the Tn10 encoded TET repressor with the tet operator is studied by thermal denaturation of the specific complexes employing operator containing purified DNA restriction fragments varying in length from 187 bp to 501 bp. Comparison of the melting curves obtained with the free DNA and DNA·repressor complexes revealed a specific stabilisation of the operator containing cooperatively melting segment in multiphasic denaturation curves. Under limiting concentrations of TET repressor the denaturation of the free DNA is observed next to the denaturation of the repressor·DNA complex. Quantitative analysis yields a binding curve with a stoichiometry of four TET repressors per tet operator containing fragment. The denaturation temperature of the complex is almost independent of the ionic strength indicating that the protein component denatures at this temperature. The half life time of the TET repressor·tet operator complex is greater than 100 min under these conditions. The tet operator on the 187 bp fragment is determined to be located between a Xba I and a Sau 3a site by removing base pairs from either end of the fragment and subsequent comparison of the melting curves. It is concluded that the TET repressor recognizes the double stranded rather than a possible cruciform structure of the tet operator. The influence of a regulatory protein on the thermal stability of a genetic control region is discussed with respect to its possible influence on the initiation of transcription.