Recent studies have suggested that wild-type p53 blocks cell cycle progression near the G1-S boundary and is involved in both differentiation and apoptosis in many types of cells including cancer cells. p53 expression is enhanced upon DNA-damaging apoptotic stimuli while Fas/Apo-1, a member of the tumor necrosis factor receptor family expressed on cell surface, transduces a signal for apoptosis upon specific ligand or antibody engagement. We demonstrated that stable transfection of the wild-type p53 gene under the control of CMV promoter induced differentiation and apoptosis under restricted conditions in cancer cells, and often caused sensitization of p53-transfected cells to Fas/Apo-1 signal. To investigate the interaction between two major apoptotic pathways involving p53 and Fas/Apo-1 we have established a system that allows to induce wild-type p53 overexpression and apoptosis in cancer cells upon treatment with anti-Fas antibody. The system also allows to investigate other factors interacting with p53 and Fas/Apo-1, and should provide a clue to understanding the biological and biochemical aspects of apoptosis.