The intradermal injection of ovalbumin in rabbits sensitized to this antigen, under circumstances resulting in the elicitation of the Arthus phenomenon, causes a systemic reaction involving alterations in leucocytes and platelets, and results in cellular thrombosis of capillaries and veins in the injected skin areas. An abnormal metabolic process develops in the injected skin areas and may be the cause of the vulnerability of these vessels to leucocyte-platelet thrombosis. The form of vascular damage determining the Arthus phenomenon is similar to that already observed in the case of the Shwartzman phenomenon, and the results of various metabolic, hematologic, and histologic studies indicate that the mechanisms resulting in both phenomena are closely related.