The sera absorbed with the particulate fraction of rabbit kidney homogenate (RK) were tested for the antibody against liver plasma membrane (LPM-Ab) by the radiometric assay method. After incubation of the isolated rabbit liver plasma membrane (RLPM) with appropriately diluted serum, IgG bound to RLPM (IgG-RLPM) was determined using 125I-labelled Staphylococcal protein A. IgG-RLPM in each subject tested was expressed in arbitrary units, that is, the multiple of the mean radioactivity associated with RLPM in 35 control subjects. Thus IgG-RLPM was 1.00 (mean) +/- 0.23 (SD) in the control subjects, 1.13 +/- 0.30 in 9 patients with chronic persistent hepatitis (CPH), 1.13 +/- 0.52 in 15 with chronic active hepatitis (CAH), 1.34 +/- 0.38 in 23 with liver cirrhosis (LC) and 3.45 +/- 0.73 in 5 with autoimmune CAH. F(ab')2 fragments from a patient with autoimmune CAH and a control subject decreased IgG-RLPM by 86.4 +/- 6.35 and -5.2 +/- 14.9%, respectively, in four patients with autoimmune CAH. LPM-Ab was detected in 0, 20.0 and 17.4% in the patients with CPH, CAH and LC, respectively. All of the patients with autoimmune CAH were positive for LPM-Ab. The absorption of the sera positive for LPM-Ab with RK decreased IgG-RLPM in various extents. In two of five patients with autoimmune CAH and two of seven patients with CAH or LC, the majority of LPM-Ab was cross-reactive with RK.