We investigated the consequences of altering the form and valence of defined autodeterminants on the initiation and spreading of experimentally induced La/Ro autoimmunity. Anti-La and Ro (SS-A) Ab responses were monitored following immunization of healthy mice with defined immunodominant and subdominant T cell determinants of the La (SS-B) autoantigen synthesized as either monomeric or multiple antigenic (MAP) peptides. Abs to mouse La (mLa) developed faster and were of higher titer in mice immunized with the subdominant mLa25-44 MAP compared with mice immunized with the 25-44 monomer. Rapid intermolecular spreading of the autoimmune response to 60-kDa Ro was observed in AKR/J mice immunized with mLa25-44 MAP, but not in mice immunized repeatedly with monomeric peptide. A/J mice immunized and boosted with the known tolerogenic mLa287-301 determinant delivered as monomeric peptide failed to develop Abs to either intact mLa or mLa287-301 peptide. However, immunization with the multivalent mLa287-301 peptide led to the rapid production of high titer mLa autoantibodies associated with a proliferative T cell response to the mLa287-301 peptide. The data suggested that the enhanced immunogenicity of MAPs was not due to augmented Ag presentation or T cell stimulation. However, MAP-, but not monomer peptide-, containing immune complexes were potent substrates for Ab-dependent fixation of complement. These results demonstrate that the form of Ag responsible for inducing autoimmunity can profoundly influence the nature and magnitude of the immune response. Thus, molecular mimicry of tolerogenic and nontolerogenic self determinants might trigger autoimmunity under conditions of altered valence.