To determine whether an antibody against hepatic sinusoidal endothelial cells was present in sera of patients with autoimmune hepatitis (AIH) type 1, we measured the serum IgG bound to the glutaraldehyde-fixed cultured rat sinusoidal endothelial cells by enzyme-linked immunosorbent assay. IgG bound to the cells was detected significantly more in patients with autoimmune hepatitis type 1 (97.1%) than in those with primary biliary cirrhosis (13.0%), chronic hepatitis C (5.9%) or B (7.9%), or healthy controls (0%). IgG-F(ab')2 fragments from autoimmune hepatitis patients also bound to the cells, and this binding was observed after absorption of the fragments with rat hepatoma cells, but not after absorption with bovine carotid endothelial cells. Culture of sinusoidal endothelial cells in the presence of IgG from AIH patients significantly reduced the number of viable attached cells. In conclusion, anti-sinusoidal endothelial cell antibody occurred in the sera from patients with autoimmune hepatitis type 1.