SUMMARYCertain antihuman lymphocyte sera raised by immunization of horses with whole thymus homogenate contain antiglomerular basement membrane (GBM) antibody and cause severe glomerulonephritis in nonhuman primates. This study was done to characterize this antibody and attempt its removal with isolated human GBM. Injection of radiolabeled antilymphocyte globulin (ALG) into rats revealed that 0.9% bound to kidney, 0.3% to liver, and 0.1% to spleen. Immunofluorescence studies showed antibody bound to the GBM in the kidney, around central veins and along sinusoids in the liver, and along sinusoids in the spleen. There was no uptake by the thymus. Use of a species with larger renal mass did not increase the percentage of antibody bound to kidney (KFAb). In contrast to in vivo studies, 5% of radiolabeled ALG was bound by GBM in vitro. When ALG was absorbed with human GBM, it no longer bound to kidney in vivo, as detected by immunofluorescence, and the KFAb content was reduced to less than 0.005%. However, its lymphocytotoxicity was retained. The results show that the anti-GBM antibody in this ALG has broad connective tissue specificity, is readily absorbed by GBM in vitro, and probably arose in response to connective tissue in the thymus homogenate used for immunization.