1. Chickens inoculated subcutaneously with 0.2 cc. of a 10–2 to 10–7 dilution of Western equine mouse brain virus had the virus in the blood serum between the 12th and the 48th hour in most instances. The fowls showed no signs of illness.
2. Viremia could be induced regularly in chickens by inoculating subcutaneously the least amount of virus which would produce encephalitis in the mouse when inoculated by the intracerebral route.
3. Even the minimal infecting dose for a chicken led to such multiplication of the virus that it was detectable in the serum in a 10–4 dilution. Moreover, a minimal infecting dose appeared to result in a longer period of viremia than was produced by a larger dose.
4. Virus has not been found to persist for more than 3 days after inoculation in any organ of the chicken tested for it and usually it did not persist over 2 days. Antibodies were present in the blood within at least 15 days after inoculation.
5. It is concluded that chickens may serve as sources of infection for mosquitoes or other blood-sucking ectoparasites for short periods of time after the infecting bite of a similar invertebrate vector. There is no evidence that the chicken serves as a latent carrier of the virus.
6. No virus could be found in the blood of 2 inoculated calves, and virus has not been demonstrated regularly or with the same case in the blood of horses or of men, as it has in that of chickens. It seems unlikely therefore that large mammals serve frequently as sources for mosquito infection.
7. These experimental data on fowls and mammals correlate well with other epidemiological and laboratory findings, in particular with the feeding preference of the mosquitoes found infected in epidemic areas.