MRIT (Mach-related inducer of toxicity)/cFLIP (cellular FADD like interlukin1-beta converting enzyme inhibitory protein) is a proteolytically inactive structural homologue of caspase-8, which is known to protect against death receptors-induced apoptosis by blocking the activation of caspase-8. We have observed that exogenous expression of MRITalpha1/cFLIP(L) isoform also protects against cell death induced by a diverse group of chemotherapeutic drugs with different mechanisms of action, including doxorubicin, etoposide, cytosine arabinoside, daunorubicin, chlorambucil and cisplatin. However, MRITalpha1/cFLIP(L) failed to protect against apoptosis induced by paclitaxel and vincristine, two microtubule-damaging agents. Although MRITalpha1/cFLIP(L) protects against chemotherapy-induced apoptosis in both solid tumor and hematopoietic cell lines, this effect was more pronounced in the former. MRITalpha1/cFLIP(L) expression is decreased during drug-induced apoptosis and exogenous expression of MRITalpha1/cFLIP(L) delays the activation of caspase-8 and -3 during drug- induced apoptosis. These results suggest that MRITalpha1/cFLIPL may be an important determinant of both death receptor- and chemotherapy-induced apoptosis and strategies aimed at downregulating its expression deserve further study as a way to overcome multidrug resistance to cancer therapy.