Rabbits were given injections of preformed complexes of purified goat antirabbit glomerular basement membrane antibody and rabbit antigoat immunoglobulin. When the animals were killed 24 hours later, examination of their kidneys revealed diffuse glomerulonephritis with widespread areas of localized loop necrosis, associated with extensive accumulations of polymorphonuclear leukocytes. By electron microscopy, accumulations of polymorphonuclear leukocytes in various degrees of disintegration were associated with areas of the basement membrane that appeared to be losing their structural integrity, suggesting that the damage to the membrane was brought about by the action of lysosomal enzymes. Reduction of the number of circulating polymorphonuclear leukocytes by pretreatment of the animals with nitrogen mustard completely prevented the lesions. Reduction of circulating complement levels by pretreatment of the animals with cobra venom factor prevented the occurrence of localized necrosis but allowed the development of the diffuse lesion.