Sera from patients with connective tissue diseases exhibit autoantibodies to a spectrum of extracellular matrix proteins. Antibodies binding to solid phase bovine fibronectin (Fn) were investigated by ELISA in patients with systemic lupus erythematosus (SLE). Sera showing binding of 2 s.d. above the mean of the normal human control were considered positive and 43/150 SLE sera (28·7%) demonstrated such binding. These antibodies were mainly IgG and IgA as determined by isotype-specific ELISA. Specificity studies on selected positive sera revealed that binding was inhibited by preincubation with soluble Fn. but not with thyroglobulin or type 1 collagen. The binding was demonstrated not to be related to interactions with rheumatoid factor, complement components or immune complexes. Additional studies to determine which Fn fragment is bound by naturally occurring anti–Fn antibodies demonstrated that the binding was predominantly to the 30-kD collagen binding domain (CBD) of Fn molecule. Inhibition studies using 120-kD, 40-kD and 30-kD Fn fragments confirmed that this binding site was the 30-kD CBD.