Active cell death induced by ligation of the Fasantigen (Fas-Ag) with its antibody, Fas ligand (Fas-L),has been known to play a major role in cell killing viaapoptosis by cytotoxic T lymphocytes (CTL). Thus, in liver transplantation, Fas-Agexpression of hepatocytes and its modification byimmunosuppressive agents such as FK 506 or CsA cantheoretically influence allograft survival. Mousehepatocytes (BALB/c) were isolated and cultured with orwithout FK 506 or CsA, and Fas-Ag expression wasdetermined by flow cytometry. Fas-Ag expression in thecontrol was 17.2 ± 2.5% after 24 hr of culture.When FK 506 or CsA was added, Fas-Ag expression withFK 506 at a concentration of 0.01-0.1 μg/ml wassignificantly lower than that with CsA (P < 0.05).When the cells were incubated with apoptosis-inducing anti-Fas-Ag monoclonal antibody, agarose gelelectrophoresis of the control cells yielded a typicalpattern of DNA fragmentations. The cells with FK 506 at0.01 μg/ml yielded the least DNA fragmentation. These findings suggested that in the in vivosetting, the hepatocytes of the allograft would have alower chance of being attacked by CTL in the hosttreated with FK 506.