The functional affinity and IgG subclass of antibodies to ss and dsDNA were measured by ELISA in five serial samples from 41 patients with systemic lupus erythematosus (SLE) who were divided into relatively homogeneous disease subgroups. Anti-dsDNA antibodies were restricted to IgG1 and IgG3 in renal disease and levels increased with disease severity. Functional affinity of IgG1 and IgG3 anti-dsDNA fell in patients with severe renal disease, suggesting that the high affinity antibody population lost from the serum was localizing in the kidneys. IgG2 anti-dsDNA were found in patients with joint and skin disease alone and in the thrombotic/spontaneous abortion subgroup. IgG2 antibody levels did not correlate with disease severity but did correlate with the presence of antibodies to Klebsiella K30 and may have represented a cross-reactive antibody population.