Autoantibodies to the recombinant extracellular domain of epidermal growth factor receptor (exEGFR) were detected by ELISA in the serum of Fas-defective old MRL/MpJ/lpr and C3H/HeJ/gld mice, but not young mice from these strains, or nonautoimmune young and old BALB/c, MRL/MpJ/++, and C3H/HeJ/MMTV mice. Compared with control human subjects without autoimmune disease, the frequency of exEGFR-binding autoantibodies was increased in scleroderma (systemic sclerosis) patients and to a lesser extent in lupus patients. Phage autoantibodies (Fv fragments) isolated from a lupus library by selection on a linear epitope of EGFR (residues 294-310) displayed the ability to bind exEGFR. Treatment of EGFR-expressing A431 cells with autoantibodies purified by affinity chromatography on immobilized exEGFR resulted in specific staining of the cells. Short-lived but strong inhibition of cellular DNA synthesis was observed in the presence of the autoantibodies. We concluded that autoantibody responses to EGFR hold the potential of fulfilling a pathogenic role in autoimmune disease.