Studies were undertaken to evaluate whether antiidiotypic antibody against antiliver-specific membrane lipoprotein is related to the disease activity of patients with hepatitis. Sera from normal individuals, patients with viral hepatitis and medical staffs were depleted of antiliver-specific membrane lipoprotein antibody by absorbing with liver-specific membrane lipoprotein-bearing cell line SK-Hep-1. Anti-antiliver-specific membrane lipoprotein was detected by binding activity to antihuman liver-specific membrane lipoprotein monoclonal antibody using solid-phase radioimmunoassay and by inhibitory activity for binding of antiliver-specific membrane lipoprotein monoclonal antibody to SK-Hep-1 using competitive radioimmunoassay. The results obtained by the two assays were equivalent for detecting anti-antiliver-specific membrane lipoprotein. Sera from patients with acute viral hepatitis during recovery phase (n = 6) and chronic persistent hepatitis (n = 12), respectively, showed significantly increased anti-antiliver-specific membrane lipoprotein activity (p < 0.01) when compared to those from normal individuals (n = 10), whereas sera from patients with chronic active liver disease (n = 12) did not reveal significantly different values from controls. Sera from the medical staff (n = 11) who had contact with hepatitis patients also demonstrated increased anti-antiliver-specific membrane lipoprotein activity (p < 0.05). Inhibitory activity of anti-antiliver-specific membrane lipoprotein in the serum appeared localized within the F(ab-)2 fragments of immunoglobulin G fraction by competitive radioimmunoassay. The specificity of the inhibitory activity of the serum was confirmed by its inability to block unrelated antigen-antibody reactions. These findings suggest that the presence of antibody against antiliver-specific membrane lipoproteins is related to the disease activity in certain patients with hepatitis.