SLE is an autoimmune disease characterized by the presence of autoantibodies against double-stranded (ds)DNA. A large proportion (approx. 40%) of patients with lupus also have increased levels of serum immunoglobulin encoded by the V(4-34) heavy chain gene, which often fluctuate with disease activity, and this gene is utilized by a subset of anti-dsDNA antibodies. In order to probe the nature of the V(4-34)-encoded immunoglobulin, B cells were isolated from the blood of two patients with active disease, using the 9G4 MoAb specific for the immunoglobulin gene product. Following cell picking, single-cell polymerase chain reaction (PCR) amplification of cDNA was used to investigate both V(H) and V(L) genes. Sequences were obtained from B cells synthesizing IgM (n = 10), IgG (n = 4) and IgA (n = 1). For V(H), all were derived from V(4-34) as expected, and the isotype-switched sequences and 2/6 IgM sequences were somatically mutated. In contrast, V(L) (12 kappa and 3 lambda) showed a low level of mutation, possibly indicating secondary rearrangements. The three most highly mutated V(H) sequences were associated with unmutated V(L) sequences. Analysis of the distribution of mutations revealed only minor clustering in complementarity-determining regions (CDRs) characteristic of antigen selection. The CDR3 lengths of V(H) ranged from five to 19 amino acids, and in 3/15 there was evidence of an excess of positively charged amino acids, compared with the normal expressed repertoire. Basic amino acids were also found at the V(L)-J(L) junctions in 4/15. These findings provide insight into the V(4-34)-V(L) gene combinations used by B cells in patients with SLE which might have clinical relevance.