Although Brucella is a good in vivo inducer of interferon, in vitro infection of murine spleen cells by Brucella suis has not, to the present time, led to in vitro synthesis. In the present work we show that normal spleen cells can however synthetize interferon in vitro when cultured together with adherent cells obtained from the spleens of syngeneic mice 45 min after in vivo inoculation. Induction and synthesis are thus shown to be distinct phenomena. Moreover soluble factors are shown to be involved in the induction phenomenon and T cells to be essential for synthesis. This in vitro brucella-induced interferon differs from in vivo brucella-induced interferon: its acid lability and its antigenic properties are characteristic of type II "immune" interferon.