Passive cutaneous anaphylatic (PCA) reactions could be elicited in young chicks using sensitizing antisera prepared in the chicken. PCA reactions were seldom observed in chickens over 4 months of age. It appears that in older chickens the antibody is not fixed to the skin. Several parameters were investigated and these included the latent period, immunological specificity, the best skin site, and the amounts of antibody and antigen necessary for consistently reproducible reactions. Extensive histological studies were carried out. Examination of the vascular morphology and the tissue infiltrate showed that the reaction was clearly a PCA reaction and not a reverse Arthus phenomenon.