In order to evaluate binding specificities of anti-F(ab′)2 antibodies from patients with systemic lupus erythematosus (SLE) and from normal healthy controls, F(ab′)2 fragments were prepared from 24 IgG myelomas with defined isoelectric points, DNA-associated idiotypes, and κ/λ light chain types. Using ELISA and hemagglutination assays, anti-F(ab′)2 antibodies from 12 healthy controls and 29 SLE patients were observed to exhibit preferential binding (λ > κ) to myeloma F(ab′)2 fragments composed of λ light chains (P < 0.0001). No correlation of anti-F(ab′)2 binding and presence of cationic, neutral, or anionic isoelectric points or for DNA-associated idiotypes on monoclonal F(ab′)2 was detected. Anti-F(ab′)2 antibodies, often elevated in SLE during remission, show preferential specificity for F(ab′)2 fragments bearing λ light chains. © 1992.