Equine immunoglobulin was detected along the glomerular basement membrane of three human homograft recipients who had been treated with equine anti-lymphocyte globulin. Anti-lymphocyte globulins, given these patients, were obtained by immunization of horses with lymphocytes from human spleens and/or lymph nodes and contained glomerular basement membrane-reactive antibodies. Quantitative paired-label isotope experiments (in rats) demonstrated that 30-170 mug/ml of kidney-fixing antibodies were present in these preparations. The anti-lymphocyte globulins formed a line of identity with a sheep anti-human glomerular basement serum when reacted against collagenase-solubilized human glomerular basement membrane in double diffusion in agar. The renal fixation of these antibodies was blocked by absorption with human glomerular basement membrane, but not by buffy-coat leukocytes, indicating that they were directed specifically toward antigens in the basement membrane and were not cross-reacting anti-lymphocyte antibodies. Anti-lymphocyte globulin preparations for human use were studied for glomerular basement membrane-reactive antibodies by a direct immunofluorescent assay in rats. Anti-lymphocyte globulin from 13 of 20 horses, and 7 of 10 serum pools from horses immunized with lymphocytes derived from solid lymphoid organs (spleen, thymus, lymph node, tonsil), contained glomerular basement membrane-reactive antibodies. Sera from 18 horses injected with thoracic duct cells or cultured lymphoblasts had no glomerular basement membrane-reactive antibodies. An equine anti-human thymus serum containing glomerular basement membrane-reactive antibodies, which produced fatal glomerulonephritis in monkeys, was shown to cause both immediate and delayed glomerular injury in monkeys after intravenous injection. The reaction of this antibody with glomerular basement membrane in vivo was associated with little complement deposition in spite of the fact that the antibody could fix complement. This lack of glomerular complement fixation resulted from almost complete in vivo decomplementation of the monkeys receiving this anti-lymphocyte globulin.