Previous studies have shown that CD4 T cells are associated with regression in primary melanoma and rejection of tumors in adoptive transfer models. The mechanism by which they mediate their anti-tumor effects remains unclear, and some studies have suggested that Fas ligand (FasL)/Fas interactions were involved. In the present study, we have examined the cytotoxic mechanism involved in CD4 T-cell killing of melanoma cells and, in particular, the role of FasL/Fas interactions in this killing. We show that the CD4 T cells in 4 clones of T cells induced apoptosis in autologous melanoma cells by MHC-restricted mechanisms but lysed an allogeneic melanoma cell by a non-apoptotic mechanism. Melanoma cells expressed both Fas and FasL, but killing of melanoma cells did not involve Fas/FasL interactions. This was shown by a lack of correlation between Fas expression and susceptibility to lysis and by failure of a monoclonal antibody to Fas to block killing by the CD4 T cells, though the latter expressed FasL. Recombinant FasL did not induce killing of melanoma cells.