The binding properties and idiotypes of a monoclonal anti-DNA antibody were studied to investigate the control of specificity in this autoimmune response. This antibody, A9, was derived from the fusion of spleen cells of an MRL-lpr/lpr mouse and the cell line X63-Ag8.653. In inhibition binding studies, A9 showed preference for single-stranded DNA and bound both ribo- and deoxyribohomopolymers. An antiserum prepared in a rabbit against A9 showed anti-idiotypic specificity after absorption with BALB/c immunoglobulins and the BALB/c myeloma MOPC 195. At least two distinct idiotypic specificities could be demonstrated by this serum, one apparently exclusive to A9 and one commonly expressed in sera of MRL-lpr/lpr mice. Serum levels of the commonly expressed A9 idiotype were also elevated in sera of C3H mice expressing the lpr gene suggesting association with B-cell activation induced by this genetic mechanism. The demonstration of a commonly expressed A9 idiotype indicates that some autoantibody variable region determinants may be genetically controlled.