The relationships between the antigen-binding specificities of four human monoclonal anti-DNA antibodies and the structural aspects of the combining sites of two of these were examined. Competition ELISAs were used to examine the reactivities of two IgM MAbs (WRI-176 and RT-79) and two IgG mAbs (D5 and B3) to a wide range of polynucleotides. The mAbs WRI-176 and RT-79 were found to bind predominantly ssDNA, with a preference for poly (dT), whilst D5 and B3 bound components of both ss- and dsDNA, and Z-DNA. The mAb B3 also exhibited a preference for A(T) rich nucleotides. Computer models were generated for the Fv regions of WRI-176 and B3. Models for RT-79 and D5 were not generated as the structure of the long CDR-H3 loops in these mAbs could not be predicted. The B3 combining site contains a groove flanked by three arginines at positions CDR-L1-27A, CDR-L2-54 and CDR-H2-53. Using interactive molecular graphics, B-DNA was docked into the B3 antigen combining site along the plane of the VH/VL interface, whilst Z-DNA was best-fitted at approximately 90 degrees to this direction. The models provide a hypothesis to explain the ability of a single autoantibody to bind two different antigens. In addition, aspects of the base specificity of B3 may be explained. The model of the WRI-176 Fv region revealed a relatively flat surface, on which a large number of hydrophobic and aromatic residues were present. Trp-H52, in particular, is prominent on the surface. This may participate in ssDNA binding through base stacking interactions. The models allow identification of potential targets for site-directed mutagenesis.