After Stokes, Bauer, and Hudson (1) found the monkey, Macacus rhesus, to be susceptible to the virus of yellow fever, several workers studied the reaction of guinea pigs to this virus. Sellards (2) was able to demonstrate the presence of the virus after three passages in guinea pigs by the transference of blood and spleen, but his results were rather irregular and could not be repeated at will. Sawyer and Frobisher (3), after giving massive intraperitoneal injections of the virus to guinea pigs, could demonstrate the virus in the blood of the animals at various intervals ranging from fifteen hours to five days after inoculation. Blood withdrawn eight or fifteen days after inoculation proved to be free from virus. These authors also showed, by protection tests in monkeys, that the guinea pig responds to an injection of virus by the development of antibodies.