The qualitative aspects of anti-DNA antibodies from the serum and those eluted from the kidney of (NZB x NZW)F1 (B/W) mice were evaluated in terms of their class and subclass specificity, avidity and isoelectric profile. Serum anti-DNA binding was demonstrated in both the IgG and IgM classes. In addition, all subclasses of IgG studied (IgG1, IgG2a, and IgG2b) were shown to bind DNA. Avidity measurements indicate that the avidity of isolated serum IgG for 125I labeled DNA decreases with age in B/W mice, while the avidity of isolated serum IgM remains relatively unchanged. Anti-DNA antibodies eluted from kidneys were found to be predominantly of the IgG1 and IgG2a subclasses. Equal amounts of both high and low avidity antibodies to DNA were demonstrated in the kidney eluates at all ages studied. Furthermore, the dissociation curves of these antibodies remained relatively unchanged with age. The isoelectric spectrum of serum antibodies to DNA obtained from mice at 4 to 11 months of age demonstrated considerable heterogeneity covering the pH range of 6 to 8.5. In contrast, the anti-DNA antibodies eluted from the kidney are slightly more basic in nature having isoelectric points between 7.2 to 9.0. This study suggests that distinct subpopulations of anti-DNA antibodies exist and that these may play an important role in the development of murine lupus nephritis.