Cytotoxic drugs commonly used in chemotherapy of leukemia and solid tumors have been shown to primarily act by inducing apoptosis in sensitive target cells. Apoptosis may involve activation of death-inducing ligand/receptor systems such as CD95 (APO-1/Fas). Treatment with anticancer drugs such as doxorubicin, methotrexate, cytarabine, etoposide and cisplatin at therapeutic concentrations leads to induction of CD95-ligand (CD95-L). CD95-L can mediate cell death in an autocrine/paracrine manner by crosslinking CD95 receptor (CD95). Interfering with CD95-ligand/receptor interaction by antagonistic antibodies to the receptor reduces sensitivity to drug-mediated apoptosis in some cell systems. In addition, treatment with cytotoxic drugs may result in upregulation of CD95, thereby increasing the sensitivity to the CD95 death signal. Apoptosis depends on activation of caspases. Deficient activation of the CD95 system was found in drug-resistant cells. In addition, CD95-resistant and doxorubicin-resistant cells displayed cross-resistance for induction of cell death. Thus, intact apoptosis pathways such as the CD95 system may play a role in determining sensitivity or resistance of tumor cells to chemotherapy.