Mice transgenic for the rat HER-2/neu oncogene (rNeu-TG) developed spontaneous breast tumors that can escape a rNeu-specific immune response induced by active specific immunotherapy (ASI). The ability of these escape tumors to grow appeared to be due to upregulation of the Fas ligand (Fas-L) molecule. In an effort to develop tools for the better elucidation of the role of Fas-L and other regulatory mechanisms in tumor escape, we established cell lines derived from escape tumors. These tumor cell lines retained MHC class I, rNeu and Fas-L expression in vitro and formed tumors in vaccinated mice. Tumor growth was accompanied by permanent Fas-L expression in vivo, both in vaccinated and control vaccinated mice, indicating that these cells have acquired constitutive Fas-L expression. Moreover, these cells induced target cell apoptosis in vitro. Thus, these cells represent a unique tool to elucidate the importance of Fas-L expressed by tumors that escaped efficient systemic immune responses. Cell Death and Differentiation (2001) 8, 687 ± 695.