Leukemic CD34(+) immature acute myeloid leukemia (AML) cells express Fas receptor but are frequently resistant to Fas agonistic reagents. Fas plays an important role in T-cell-mediated cytotoxicity, and recently it has been suggested that altered Fas signaling may contribute to drug resistance. Therefore, Fas resistance could be one of the mechanisms by which AML progenitors escape chemotherapy or T-cell-based immune intervention. However, the molecular mechanism of Fas resistance in AML cells has not been identified. Fas signaling can be interrupted at 3 mains levels: Fas clustering, alteration of death-inducing-signaling-complex (DISC) formation, and effector caspase inhibition of downstream caspase-8. This study shows that in the Fas-resistant CD34(+)CD38(-) KG1a cells, Fas agonists resulted in Fas aggregation but not in caspase-8 activation, related to a defect in DISC formation. However, pretreatment with chelerythrin, but not with calphostin C, resulted in the restoration of Fas-induced caspase-8 activation and cytotoxicity, suggesting that some atypical protein kinase C (PKC) isoforms contributed to the lack of DISC formation. Indeed, treatment with antisense oligonucleotides directed against PKC zeta and enforced expression of Par-4, a negative regulator of PKC zeta activity, restored Fas-induced caspase-8 activity and apoptosis. Moreover, it was found that PKC zeta interacts with FADD and that PKC zeta immunoextracts prepared from KG1a cells are able to phosphorylate FADD in vitro, whereas this phosphorylation is dramatically reduced in Par-4 transfectant cells. In conclusion, it is suggested that in AML cells, PKC zeta plays an important role in Fas resistance by inhibiting DISC formation, possibly by phosphorylating FADD.