Complexes between the Lac repressor and a small DNA operator fragment (29 base pairs) were investigated using polyacrylamide gel electrophoresis and solution X-ray scattering. Titration of the DNA fragment with the repressor, followed by gel electrophoresis showed that only two types of complexes are formed with repressor/operator ratios of 0.5 and 2. Radii of gyration and forward scattered intensities were obtained from Guinier plots for repressor/operator ratios ranging from 0.3 to 2. They demonstrated that the first complex contains one repressor and two operators, whereas the second one contains four repressors and two operators. Mixing operator and repressor in equimolar concentrations leads to a mixture of both complexes. A possible model for the four repressor/two operator complex is proposed.