Anti-DNA antibodies are autoantibodies unique to systemic lupus erythematosus. Studies of their structure have demonstrated cross reactive idiotypes present in genetically unrelated individuals. Despite much research, it is still not clear what triggers their production and what governs the presence of particular idiotypic determinants in their structure. To study the role of genetic and environmental factors in the expression of idiotype, we analyzed sera of SLE patients, their family members and nonautoimmune individuals vaccinated with pneumococcal polysaccharide, for the presence of the 8.12 idiotype, which is present on lambda light chains of anti-DNA antibodies. Elevated titers of the 8.12 idiotype was found in the serum of 57% of SLE patients. Elevated titers were present in only 9% of family members, and always associated with the presence of high levels of IgG anti-DNA antibodies. Following vaccination with pneumococcal polysaccharide, 8.12 reactive anti-pneumococcal antibodies were produced by 7 of 10 non-autoimmune individuals and 8.12 reactive anti-DNA antibodies by one. These results suggest that 8.12 reactive antibodies are antigen driven and bind structurally related antigens, but there is no evidence that expression of this idiotype is genetically controlled.