The importance of the idiotypic network is represented in experimental SLE induced by active immunization of naive mice with an anti-DNA idiotype (Ab1) emulsified in adjuvant. The mice after 4 months of incubation generate Ab3 having anti-DNA activity. In addition, the mice develop other serological markers for SLE associated with clinical and histopathological manifestations characteristic of the disease. To confirm further the etiological role of the idiotype in this experimental model, the mice were treated with specific anti-idiotypic antibodies (anti-Id) which were also conjugated to a toxin-saporin (Immunotoxin (IT)). Pretreatment of hybridoma cell line producing the anti-anti-Id (anti-DNA = (Ab3)) for 48 h with the anti-Id MoAb (Ab2) reduced the production of anti-DNA by 58%, while pretreatment with the IT resulted in 86% decrease in anti-DNA secretion (saporin alone had only 12% effect). The anti-Id MoAb had no effect on the production of immunoglobulin by an unrelated cell line. In vivo treatment of mice with experimental SLE led to a significant decrease in titres of serum autoantibodies, with diminished clinical manifestations. The results were more remarkable when the IT was employed. These suppressive effects were specific, since an anti-Id treatment of experimental anti-phospholipid syndrome was of no avail. The anti-Id effect was mediated via a reduction in specific anti-DNA antibody-forming cells, and lasted only while anti-Id injections were given. Discontinuation of the anti-Id injection was followed by a rise in titres of anti-DNA antibodies. No immunological escape of new anti-DNA Ids was noted. Our results point to the importance of pathogenic idiotypes in SLE and to the specific potential of implementing anti-idiotypic therapy, enhanced by the conjugation of the anti-Id to an immunotoxin, in particular one with low spontaneous toxicity.