The structure of complexes formed between cAMP receptor protein (CRP) and various restriction fragments from the promoter region of the lactose operon has been analysed by measurements of electrodichroism. Binding of CRP to a 62-bp fragment containing the major site leads to an increase of the rotation time constant from 0.33 to 0.43 microseconds; addition of cAMP to the complex induces a decrease to 0.25 microseconds. Similar data are obtained for a 80-bp fragment containing the operator site; however, in this case the decrease of the rotation time for the specific complex is only observed when the salt concentration is increased from 3 to 13 mM. A 203-bp fragment containing both sites showed a corresponding change after pre-incubation at 50 mM salt. The salt dependence of the rotation time for the specific complex formed with the 203-bp fragment also indicates that a compact structure is formed at 13 mM salt, whereas the structure is not as compact at 3 mM salt. A 98-bp fragment without specific CRP sites did not reveal changes corresponding to those observed for the specific fragments. The rotation time constants together with the dichroism amplitudes indicate that binding of CRP to specific sites in the presence of cAMP leads to the formation of compact structures, which are consistent with bending of DNA helices. The observed strong salt dependence of the structure is apparently due to electrostatic repulsion between adjoining helix segments.