One of the first steps in the development of cerebral toxoplasmosis is the penetration of the blood-brain barrier, which is comprised of microvascular endothelial cells. We examined the capacity of human brain microvascular endothelial cells (HBMEC) to interact withToxoplasma gondii. We found that stimulation of HBMEC with gamma interferon (IFN-γ) resulted in the induction of toxoplasmostasis. The capacity of HBMEC to restrictToxoplasma growth after IFN-γ stimulation was enhanced in the presence of tumor necrosis factor alpha (TNF-α). In addition, we found that IFN-γ induced a strong induction of indoleamine 2,3-dioxygenase (IDO) activity in HBMEC, and this enzyme activity was enhanced by costimulation with TNF-α. The addition of excess amounts of tryptophan to the HBMEC cultures resulted in a complete abrogation of the IFN-γ–TNF-α-mediated toxoplasmostasis. We therefore conclude that IDO induction contributed to the antiparasitic effector mechanism inducible in HBMEC by IFN-γ and TNF-α.