The eye is a privileged site that cannot tolerate destructive inflammatory responses. inflammatory cells entering the anterior chamber of the eye in response to viral infection underwent apoptosis that was dependent on Fas (CD95)-Fas ligand (FasL) and produced no tissue damage. In contrast, viral infection in gld mice, which lack functional Fast, resulted in an inflammation and invasion of ocular tissue without apoptosis. Fas-positive but not Fas-negative tumor cells were killed by apoptosis when placed within isolated anterior segments of the eyes of normal but not Fast-negative mice. Fast messenger RNA and protein were detectable in the eye. Thus, Fas-FasL interactions appear to be:an important mechanism for the maintenance of immune privilege.