Human glomerular basement membrane (GBM) emulsified in Freund's complete adjuvant was injected into 16 Holtzman rats. Eleven (69%) of them subsequently developed glomerulonephritis. This disease was characterized by glomerular changes which included focal lobular hypercellularity due to localized hyperplasia of intracapillary (mesangial or endothelial) cells which often obstructed glomerular capillaries. Later, affected lobules showed necrosis of intracapillary cells, and capillary lumina were filled with homogeneous eosinophilic material. An immunoperoxidase technique revealed autologous anti-GBM antibody in all 3 layers of the GBM and in some regions it was present in greater amount in the lamina rara interna and externa than in the lamina densa. Experimental auto-immune glomerulonephritis in the rat is thus characterized morphologically by a focal proliferative glomerulonephritis which proceeds to focal glumerulosclerosis.