Fas antigen is constitutively expressed in the normal colon epithelium, but considerably diminished in most colorectal carcinomas. In the present study, we examine the relationship between Fas antigen expression and apoptosis using the colorectal carcinoma cell line COLO 201, on which a low grade of Fas antigen is expressed. Anti-Fas antibody had no effect on the induction of apoptosis of COLO 201. However, TNF-α and/or IFN-γ, independently and additively, up-regulated Fas antigen expression on COLO 201 and induced apoptosis in a dose-dependent manner. Both cytokines also increased the COLO 201 sensitivity to anti-Fas antibody, resulting from the down-modulation of Bcl-2 and the up-regulation of Bax. These findings indicate that cytokine(s) plus anti-Fas antibody (which mimics natural Fas ligand) are more effective in inducing apoptosis of COLO 201 than cytokine(s) alone. These findings suggest that immunotherapy in combination with cytokine(s) and lymphokine-activated killer (LAK) cells will become a more effective therapy for cancer than cytokine(s) or LAK cells alone, since the Fas ligand is expressed on activated T cells, natural killer cells and macrophages.