Passive cutaneous anaphylaxis was produced in chickens with bovine serum albumin (BSA) and anti-BSA chicken serum. Colloidal carbon was then given intravenously to identify the leaky vessels. Examination of the resulting sequential changes revealed a marked increase in vascular permeability affecting the venules. A noteworthy feature of the reaction was the early participation of the basophils, which contained phagocytosed carbon particles. Eosinophils were absent.