The structural features of the interaction between DNA and lupus anti-DNA antibodies were investigated by competition experiments with low molecular weight synthetic compounds. It was found that two correctly spaced chemical components, a substituted aromatic ring system and a negatively charged acidic residue, were required for the binding of most anti-DNA autoantibodies to their respective antigens. These chemical elements are combined in the structure of several anionic dyes, including some certified food colours. The dyes were found to interfere efficiently and specifically with the lupus DNA-anti-DNA immune reaction. Therefore, this family of compounds may serve as a basis for the development of a new approach to drug therapy in SLE.