The role of polymorphs in the Arthus type hypersensitivity vasculitis has been studied. Polymorphs were found to play an essential role in not only producing the inflammatory vasculitis, but also were instrumental in ridding the damaged vessel of the antigen, probably by means of proteolytic catabolism at the inflammatory site. A temporal relationship between the disappearance of antigen from the damaged vessels and a decrease in inflammatory reaction was found. The earliest localization of antigen and its associated rabbit globulin in the Arthus vasculitis was found to be beneath the endothelium of small vessels. Since submitting this article for publication, Sorkin and Boyden (J. Immunol., 1959, 82, 332) have reported the catabolism of antigen in the presence of antibody by mononuclear cells obtained from the peritoneal cavities of guinea pigs. Evidence was presented indicating that the antigen molecule was actually broken down by the mononuclear cells. Techniques similar to those reported here were used.