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Purified P30, the principal iodinatable membrane protein of Toxoplasma gondii, induced proliferation of peripheral blood mononuclear cells from seropositive individuals but not from seronegative individuals. Culture supernatants from stimulated cells of seropositive individuals blocked the growth of T. gondii in human fibroblasts, whereas those from antibody-negative individuals failed to do so. The anti-toxoplasmic effect of culture supernatants correlated with the induction of indoleamine 2,3-dioxygenase and the destruction of tryptophan, as previously described for fibroblasts treated with recombinant γ interferon (IFN-γ). The anti-toxoplasmic effect was blocked by monoclonal antibody to IFN-γ. The protective effect correlated with the amount of IFN-γ in the culture supernatant, as measured by inhibition of viral CPE. Thus, the level of IFN-γ appears to be an important immune factor in protection against toxoplasmosis in humans. [ABSTRACT FROM PUBLISHER]