To probe the mechanism of spontaneous formation of anti-DNA antibodies, we studied the isotype distribution of anti-DNA antibodies in the plasma of NZB/W mice with incipient, chronic, and drug-attenuated nephritis. The concentration of anti-DNA antibodies in plasma did not discriminate between the various groups of mice, and the anti-DNA activity in renal eluates was very low and did not reflect the course of nephritis. Progression of incipient to chronic nephritis was associated with increase, and drug attenuation of nephritis with decrease, in plasma concentration of all isotypes tested. Anti-DNA antibodies were detected in all classes (IgG, IgM) of antibodies studied and the anti-DNA antibodies were found to be unrestricted with respect to IgG isotypes and within a given (IgG2a) isotype. The data indicate that the spontaneous immune response to DNA in NZB/W mice reflects activation of several autoreactive clones and that, overall, anti-DNA activity in the plasma or renal eluate is a poor predictor of extent of renal disease.