Kidney allografts were transplanted through cervical blood vessels to dogs, normal or with induced glomerulonephritis (those producing antirenal antibodies). Fifteen minutes after the introduction of the allogenic kidney into the recipient's circulation, a sharp decline in the anti-kidney antibody titer was observed in the blood in flowing out of the transplanted kidney. It was accompanied by a lowering in the platelet count, and by a decrease in the content of fibrinogen and complement. Considerable amount of FDP appeared. A high degree of correlation was found between the immunological alterations and hemocoagulative indices. Examined by means of light microscopy, allograft tissue samples, withdrawn three hours after the transplantation, demonstrated the typical hyperacute rejection phenomen: fibrin deposition in the capillaries of the glomeruli, and fibrinoid impregnation of the walls of large and medium size interstice arteries. Immunoglobulins eluted from the transplants precipitated kidney antigen specifically, and if fluorescent-dye-labeled, caused luminescence in the capillar loops of glomeruli and the walls of larger vessels. These findings suggest an explanation for acute kidney allotransplant damages occuring soon after the operation in patients with glomerulonephritis even in the absence of preexistent lymphocytotoxic antibodies.