Induction of circulating interferon and protection against vaccinia virus infection in mice by injection of Brucella abortus were studied. It was demonstrated that morphologically intact brucellae (either live or killed by heat or exposure to NaOH) induce high and prolonged levels of circulating interferon in mice. In each instance, the inducing principle remained associated with the bacterial particle. Disruption of brucellae by mechanical means destroyed the interferon-inducing capacity. However, by alkalinization of the water extract of disrupted bacilli, an interferon inducer could be rescued. On intravenous injection, this inducer caused a typical endotoxin type of interferon response with a peak value at 2 hr. Mice pretreated with cycloheximide showed an enhanced interferon response to the brucella extract, but a reduced reaction to live brucellae. The significance of these data, in relation to the triggering of de novo interferon synthesis by brucella, is discussed. It was also observed that small doses of brucellae protected mice for at least 1 month against vaccinia virus infection. High doses of heat-or alkali-killed brucellae protected the animals for only a short time, and disrupted brucellae did not afford any protection. Thus, there was a good correlation between interferon-inducing capacity and short-term protective activity. Long-term protection, on the other hand, seemed to be related to multiplication of brucellae in the body.