The gamma interferon (IFN-gamma) production and the cell populations participating to this production were examined in Toxoplasma-infected mice. When spleen cells from Toxoplasma-infected mice were cultured with Concanavalin A (Con A) or OK-432, a Streptococcal preparation, they produced significantly high levels of IFN-gamma as compared with that of noninfected mice. Such enhanced IFN titers were observed as early as at 5 days postinfection, reached at the maximum levels on 20 days around and declined gradually thereafter. Treatment of spleen cells from the infected mice with either monoclonal anti-Thy-1.2 antibody plus complement or macrophage-blocking agents virtually abolished the IFN production. The spleen cells producing IFN-gamma were more susceptible to the treatment with monoclonal anti-Lyt-1.2 than anti-Lyt-2.2 antibodies, suggesting that CD4+ T cells are main producers of this lymphokine. When mice infected with Toxoplasma 10 days previously were injected with lipopolysaccharide (LPS), a well-known inducer of IFN-alpha/beta, the sequential production of IFN-alpha/beta and IFN-gamma was induced in their circulation.