Earlier experiments had shown that the sera of animals immunized with active virus have much greater protective potency when serum-virus mixtures are injected intraperitoneally into 12 to 15 day old mice than when given intracerebrally. The present work was concerned with similar tests on sera derived from guinea pigs immunized by vaccines in which the virus had been inactivated by formalin.
In comparing the content of antiviral body by means of intracerebral and by intraperitoneal inoculation, it was found that both sera show about the same low degree of neutralizing capacity by the former method. By intraperitoneal inoculation, on the other hand, serum collected from guinea pigs immunized by means of active virus reveals high protective power, while that from animals receiving formolized, inactive virus exhibits lower neutralization titers which approach those obtained by the intracerebral method. The significance of this unexpected finding is discussed.