Acute glomerulonephritis was produced in rabbits by single intravenous injections of bovine albumin. Characteristic subepithelial deposits, with or without preservation of the lamina rara externa, were seen in all rabbits as early as five days prior to and as late as three weeks after antigen clearance. An increase of mesangial cells was associated with increasing deposition of matrix. The latter appeared as electron-translucent trabeculae, which gradually increased in density with deposition of microfibrils. These frequently were found in continuity with similar intracellular structures. This seems to support the hypothesis that the mesangial cells contribute to the deposition of matrix. In most rabbits much glycogen was recognizable in the mesangial cytoplasm. There is no correlation between the severity of glomerulonephritis discernible by light microscopy and subepithelial deposits seen by electron microscopy. The mesangial changes did not resolve within four weeks of observation, and the deposits did not disappear completely within this period. Acute serum-sickness glomerulonephritis in rabbits closely resembles acute glomerulonephritis in man.