Toxoplasrna gondii, an obligate intracellular parasite, is able to replicate in human brain cells. We recently showed that interferon (1FN)-y-activated cells from glioblastoma line 86HG39 were able to restrict Toxoplusma growth. The effector mechanism responsible for this toxoplasmostatic effect was shown by us to be the IFN-y-mediated activation of indolamine 2,3-dioxygenase (IDO), resulting in the degradation of the essential amino acid tryptophan. In contrast, glioblastoma 87HG31 was unable to restrict Toxoplasma growth after IFN-y activation, and IFN-y-mediated I D 0 activation was weak. We observed that tumor necrosis factor (TNF)-a alone is unable to activate I D 0 or to induce toxoplasmostasis in any glioblastoma cell line tested. Interestingly, we found that TNF-a and IFN-y were synergistic in the activation of I D 0 in glioblastoma cells 87HG31,86HG39 and U373MG and in native astrocytes. This was shown by the measurement of enzyme activity as well as by the detection of I D 0 mRNA in TNF-a + IFN-y activated cells. This I D 0 activity results in a strong toxoplasmostatic effect mediated by glioblastoma cells activated simultaneously by both cytokines. Antibodies directed against TNF-a or IFN-y were able to inhibit I D 0 activity as well as the induction of toxoplasmostasis in glioblastoma cells stimulated with both cytokines. Furthermore, it was found that the addition of L-tryptophan to the culture medium completely blocks the antiparasitic effect. We therefore conclude that both TNF-a and IFN-y may be involved in the defense against cerebral toxoplasmosis by inducing I D 0 activity as an antiparasitic effector mechanism in brain cells.