It is demonstrated that BALB/c/DUB mice receiving chronic intraperitoneal injection of low doses of murine native dimeric or denatured multimeric or monomeric serum fibronectin in the absence of adjuvant produce a humoral autoimmune response to mouse fibronectin and have significant levels of circulating immune complexes. With the use of the Western immunoblotting technique, sera or mice immunized with native or sodium dodecyl sulfate (SDS)-treated fibronectin reacted with native, reduced and alkylated, and heat- and SDS-treated fibronectin. Sera from mice injected with reduced and alkylated fibronectin, however, reacted preferentially with reduced and alkylated fibronectin. In a competitive inhibition ELISA, the reactivity of immune sera from mice immunized with reduced and alkylated fibronectin to insolubilized reduced and alkylated fibronectin could be inhibited (66%) by preincubation with soluble reduced and alkylated fibronectin, but not by preincubation with native or SDS-treated fibronectin (<25%). Glomerular mesangial electron-dense deposits were observed in animals given injections of all three forms of fibronectin, but not in saline-injected animals. Indirect immunoperoxidase studies suggested that these dense deposits were at least partially immune in nature, because the authors were able to demonstrate binding in vivo of mouse IgG.