Summary and Conclusions: A study of the mechanism of the Arthus phenomenon, utilizing a purified protein for sensitization and the collodion particle agglutinative method for determining the precipitative titer of the serum, has revealed the following facts: In every instance in which the Arthus phenomenon was produced, specific precipitins were likewise demonstrated in the serum at the time of the cutaneous test.There is a definite parallelism between the precipitative potency of the serum and the intensity of cutaneous reactivity. The only instance of an exception was in young animals, where cutaneous reactivity to inflammatory stimuli in general is naturally low.Reduction in precipitative titer of an animal's serum by massive injection of antigen (partial desensitization) caused a marked decrease or disappearance of cutaneous hypersensitivity which, however, reappeared concomitantly with the subsequent rise in precipitative titer.Cutaneous hypersensitivity of rabbits passively sensitized by injection of serum from rabbits strongly hypersensitive, varied directly with the amount of serum injected. When the specific precipitins were removed from such sera by antigenic absorption, their capability of conferring passive sensitization disappeared.The Arthus phenomenon is dependent upon the union within tissues of circulating precipitin and its specific antigen.