Fas ligand (FasL/CD95L) is best known for its role in delivering apoptotic signals through its receptor, Fas (APO-1/CD95). In this study, we present evidence that FasL has a second role as a signaling receptor. Alloantigen-specific proliferation by multiple FasL- murine CTL lines is depressed compared to that of FasL+ CTL lines. FasL- CTLs kill efficiently on a per recovered cell basis and can achieve wild-type levels of proliferation upon stimulation by optimal doses of anti-CD3, suggesting the lack of a costimulatory signal during antigen stimulation. To test this hypothesis directly, soluble FasIgG, a fusion protein of murine Fas and human IgG1, was added to FasL+ CTLs to demonstrate that blocking cell surface Fas-FasL interactions mimics the depression observed for FasL- CTLs. In addition, plate-bound FasIgG in conjunction with suboptimal anti-CD3 stimulation augments proliferative signals in FasL+ but not FasL- CTLs. In contrast to these results with CD8+ T cells, alloantigen-stimulated FasL- CD4+ T cells proliferate vigorously compared to FasL+ cells. These data demonstrate that reverse signaling through FasL is required for CTLs to achieve maximal proliferation and may provide clues to differences in the homeostatic regulation of activated CD4+ and CD8+ T cells during an immune response.