Previous studies from this laboratory (1, 2) have indicated that certain foreign protein antigens, among them bovine "y-globulin, injected into the blood stream of mice, may persist in the blood for about 8 weeks and in certain tissues for nearly 14 weeks. It is to be recalled that the detection of minute traces of antigen, persisting in the blood or tissues of these mice, was carried out at various intervals after the injection by transferring blood or ground tissues from these animals, as "donors," to the peritoneal cavities of normal, "recipient" mice which were challenged 2 days later by an intravenous injection of a strong, anti-bovine 7-globulin rabbit serum. If antigen was present in the transferred materials and had been absorbed by the recipients in suflident amounts to sensitize them, certain specific vascular responses of reversed passive anaphylaxis appeared in the smaller, and sometimes too in the larger, blood vessels of the ears when the latter were examined under the microscope. These reactions have been termed "ear vascular responses, EVR." In mice receiving very minute amounts of antigen the specific EVR, though weak, can be distinguished from other, non-specific vascular responses.The persistence of antigen, as indicated by this sensitive test (1), the "mouse transfer test," appears to be longer than that found by many workers who have investigated the persistence of antigens by other means and in other species. The work of some authors (3-24), however, has pointed to a long persistence of certain antigens in the rabbit, guinea pig, and man. Hence the question arose whether the long persistence of injected antigen in mice (1) is a species' peculiarity. Mice do not form precipitins well, and in consequence injected foreign antigens might remain intact for relatively longer periods than in other animals if not subjected to the influence of high concentrations of circulating antibody.To investigate this possibility the persistence of an antigen was studied in animals which form antibody relatively well, employing the same general plan of attack as in the previous work with mice (1). That is to say, bovine 341 on May 12, 2018 jem.rupress.org Downloaded from