Background: During carcinogenesis, tumors develop multiple mechanisms for evading the immune response, including upregulation of Fas ligand (FaL/CD95L) expression. Expression of FasL may help to maintain tumor cells in a state of immune privilege by inducing apoptosis of anti-tumor immune effector cells. Recently this idea has been challenged by studies reporting that tumor cells of varying origin do not express FasL. In the present study, we aimed to comprehensively characterize FasL expression in tumors of both murine and human origin over a 72 hour time period. Methods: RNA and protein was extracted from six human (SW620, HT29, SW480, KMI 2SM, HCT116, Jurkat) and three mouse (CMT93, CT26, B16F10) cancer cell lines at regular time intervals over a 72 hour time period. FasL expression was detected at the mRNA level by RT-PCR, using intron spanning primers, and at the protein level by Western Blotting and immunofluorescence, using a polyclonal FasL- specific antibody. Results: Expression of FasL mRNA and protein was observed in all cell lines analysed. However, expression of FasL mRNA varied dramatically over time, with cells negative for FasL mRNA at many time points. In contrast, 8 of the 9 cell lines constitutively expressed FasL protein. Thus, cells can abundantly express FasL protein at times when FasL mRNA is absent. Conclusion: These findings demonstrate the importance of complete analysis of FasL expression by tumor cells in order to fully characterize its biological function and may help to resolve the discrepancies present in the literature regarding FasL expression and tumor immune privilege. © 2006 Ryan et al; licensee BioMed Central Ltd.