An idiotope designated 9G4 (9G4^Id) is known to be a marker for immunoglobulins which utilize a particular VH gene, V_H4–21. The idiotope has been found to be present on anti-DNA antibodies, and have been identified in 45% of sera from patients with SLE. This idiotope is strongly associated with lupus being very uncommon amongst the other autoimmune rheumatic diseases tested. This distinction is unlike virtually any of the other DNA antibody idiotypes described which are much more widely distributed. 9G4^Id levels were found to fluctuate with disease activity in some lupus patients and this idiotope was detected in 3/11 SLE renal biopsies tested. Its presence is associated with the HLA markers A1 and B8 and raised 9G4^Id levels are not simply a reflection of hypergammaglobulinaemia. Thus a new DNA antibody associated idiotope has been identified. Expression of the idiotope indicates that a notable proportion of anti-DNA antibodies have VH segments encoded by the same, or closely related genes, and that these restricted immunoglobulins are involved in the renal pathology found in SLE.