To investigate the effect of all-trans retinoic acid (ATRA) and 9-cis retinoic acid (9-cis RA) on the expression of Fas and the sensitivity to anti-Fas induced apoptosis in HL-60 cells.Fas expression was detected by flow cytometry. Anti-Fas induced apoptosis (AFIA) was determined by morphological features, DNA fragment electrophoresis, and flow cytometric cell cycle analysis.Fas antigen was weakly expressed on HL-60 cells. After incubation of HL-60 cells with 10(-6) mol/L ATRA or 9-cis RA for 4 days, both the Fas expression and AFIA were significantly enhanced. 9-cis RA was more potent than ATRA in increasing the sensitivity to AFIA.RA might upregulate Fas expression. Further elucidation of its molecular mechanisms might provide a rationale for new therapeutic strategies of malignant diseases.