Summary Levels of circulating virus and interferon were followed in chickens inoculated intravenously with Powassan virus. A massive dose of virus (109.3 mouse LD50) given to 6-day-old chicks resulted 3 to 4 days later in virus titers lower than those developed by chicks of the same age receiving only 102.9 mouse LD50. Circulating interferon was detected in the former group at 4 and 6 hours following inoculation but at no time was it found in birds given the smaller dose. Interferon appeared more rapidly and in higher titer in the serum of 3-month-old chickens inoculated with 1010.1 mouse LD50 of virus.