Since we previously observed a remarkable decrease in the numbers of T cells in the Peyer's patches of the small intestines in C57BL/6 mice following peroral infection with Toxoplasma gondii, we performed studies to examine the mechanism(s) whereby this decrease in numbers of the T cells occurs. We found that apoptotic cell death of CD4+ and CD8+ alphabeta T cells occurred in Peyer's patches following infection. Upregulation of Fas expression was observed in these T cells. C57BL/6-background mutant mice which lack functional Fas antigen did not develop apoptosis in their Peyer's patches following infection. Treatment of infected C57BL/6 mice with anti-gamma interferon (IFN-gamma) monoclonal antibodies prevented the upregulation of Fas on their Peyer's patch T cells and inhibited the occurrence of apoptosis of these T cells. These results indicate that IFN-gamma induces Fas-dependent apoptosis in CD4+ and CD8+ alphabeta T cells in Peyer's patches in C57BL/6 mice following peroral infection with T. gondii.