The intravenous injection of one large dose of serum into rabbits caused a variety of changes of considerable complexity. 1. There was an immediate proliferation of mesenchymal cells, including plasma cells, particularly in heart, lungs, and spleen. 2. The signs of serum disease developed only at the time of abundant antibody formation, before significant quantities of antibody were laid down in the vascular connective tissue. 3. Allergic arteritis, marked glomerular nephritis, myocardial necrosis, and Aschoff body-like structures were seen only after hypersensitivity had developed. It appears that most, if not all, of these pathological alterations were true Arthus phenomena. 4. There were at least two distinct varieties of allergic arteritis and glomerular nephritis, namely a proliferative one, following the first injection, and a a necrotizing one, seen only after 2 injections. It appears that the first was a subacute Arthus phenomenon, while the latter was an acute Arthus phenomenon superimposed on a subacute one. The subacute experimental glomerular nephritis resembled the intracapillary glomerulonephritis in man, while the acute variety was like human extracapillary glomerular nephritis.