A short term model in which circulating antigen-antibody complexes and host complement localized in vessel walls of guinea pigs was analyzed. Localization was accomplished by subjecting the animals to anaphylactic shock. The circulating macromolecules, such as antigen-antibody complexes, appeared to localize by being trapped in the vessel wall along the basement membrane that acted as a filter during a state of increased permeability of the vessel. It was suggested that this point of localization between the endothelial cell and the basement membrane may well represent the earliest focus of inflammation in diseases caused by the deposition of injurious macromolecules such as soluble antigen-antibody complexes from the blood stream. Complexes localized in the vessel walls did not provoke Arthus-type vasculonecrotic reactions even though in both these vessels and in cutaneous Arthus reactions antibody, antigen, and host complement (C'3c) were deposited in the vessel walls. The possibility was presented that since circulating macromolecules and probably complexes deposited in (a) relatively small amounts, and (b) in a position beneath endothelial cells, they were not strongly chemotactic toward circulating polymorphonuclear leukocytes. Vasculonecrotic reactions, therefore, were not observed. It was brought out that this may be similar to the situation in glomerulonephritis induced by localized immune complexes, in which severe necrosis is not observed. In the Arthus vascular reaction, host complement was found microscopically accumulated with the immune reactants in affected vessel walls.