Mesangial cells of F344 rats degenerated and then disappeared within 2 days after the intravenous administration of rabbit antirat thymocyte serum (ATS). Rabbit IgG and rat C3 were identified in the mesangium in the rat glomeruli. To establish the glomerular binding site of ATS administered intravenously into rats, one kidney of each rat given ATS intravenously 12 h earlier was perfused ex vivo through the renal artery with peroxidase-labeled antirabbit IgG followed by sequential glutaraldehyde and diaminobenzidine perfusions to minimize the ultrastructural damage. The other kidney was removed before the perfusion for histologic study to examine the glomerular injury. The rabbit IgG identified by peroxidase-reaction product was present diffusely in the glomerular mesangium when viewed by light microscopy and exclusively on the surfaces of most mesangial cells by electron microscopy. Immunofluorescence microscopy showed rabbit IgG essentially in the mesangium, and electron microscopy revealed the degeneration of mesangial cells in the kidneys that had been removed before the surgical perfusion. However, no histological abnormalities were found in the kidneys from control rats given ATS absorbed with rat thymocytes. The present study showed that the intravenous administration of ATS into rats induced the extensive mesangial cell damage by the binding of ATS to Thy-1 antigens on the mesangial cells.