Mice, chickens, hamsters, rats and monkeys were studied for induction of circulating interferon following viremias resulting from infection or intravenous injection of diverse viruses. Most combinations of animal species and virus types resulted in early production of serum interferon, although not all such combinations were successful. These results indicate that circulating interferon is a rather general response by animals during the viremic phase of virus infection. Interferon was observed to leave the serum in vivo at a rate of about thirtyfold/hr. Using this rate it can be conservatively estimated that a total of 100,000 units of circulating interferon are produced during many of the viremias of mice.