Objective To investigate the relationship between expression of active form of caspase-3 and cell cycle in Fas-mediated apoptosis of B lymphocytoma cell line MML-1. Methods MML-1 cells were incubated with agonistic anti-Fas antibody for different time,and cell apoptosis was induced.Cell apoptotic rates were analysed by flow cytometry,and sensitivity of MML-1 cells to apoptosis was determined.The expression of active form of caspase-3 was analysed by double staining with PI-Triton X and FITC-active caspase-3.Cyclin A,B_1 and E were selected as cell cycle markers for S,G_2/M and G_1 phase of MML-1 cells,and the expression of active form of caspase-3 was detected by flow cytometry. Results The cell apoptotic rate reached 56% after induction by Fas for 6 h.After induction by Fas for 4 h,the active form of caspase-3 was mainly expressed in cells of G_1 phase,while rarely in cells of S and G_2/M phase.Cells with negative cyclin A and B_1 and positive cyclin E expressed active form of caspase-3. Conclusion The expression of active form of caspase-3 in MML-1 cells mediated by Fas might be cell cycle dependent.Cells entering into late G_1 and early S phase first express active form of caspase-3,and their sensitivity to Fas-mediated apoptosis is the highest.