Abstract： Objective To explore the effect of bone marrow infusion in combined with costimulatory signal system blockade on the survival time of grafted skin in rats and its possible mechanisms.Methods Lewis rats as receptors received 2 × 108 bone marrow cells from inbred BN rats through tail vein on day 0 and anti-CD 25 monoclonal antibody on day 0,day 2,day 4,day 6,and day 8. The receptors also received CTLA4Ig (CTLA4Ig group), anti-CD 154 monoclonal antibody (anti-CD 154 monoclonal antibody group),CTLA4Ig and anti-CD 154 monoclonal antibodies (combined treatment group) in accordance with the requirements of each group. Each group received skin graft from BN rats on day 8. There was also control group in which receptors just received skin grafting. The symptom of GVHD,chimerism rate in peripheral blood, apoptosis of T cells and the survival time of grafed skin were observed. Results GVHD was not observed in all groups. The chimerism could be observed in CTLA4Ig group,anti-CD 154 monoclonal antibody group and combined treatment group on day 7. On day 21 chimerism could also be detected. The chimerism rate in combined treatment group was significant higher than that in other groups (P＜0. 01 ). In CTLA4Ig group, anti-CD 154 monoclonal antibody group and combined treatment group, there was no significant difference in apoptosis of T cells on day 7 and day 21, but there was significant difference between experimental groups and control group (P＜0. 05,P＜0. 01). The survival time of grafted skins in CTLA4Ig group,anti-CD 154 monoclonal antibody group and combined treatment group was significant longer than that in control group (P＜0. 01). The survival time of grafted skins in combined treatment group was (16. 7±3. 1) days,which was significant longer than that in CTLA4Ig group and anti-CD 154 monoclonal antibody group (P＜0. 05). Conclusion Bone marrow infusion in combined with costimulatory signal system blockade prolonged the survival time of grafted skin,and apoptosis of T cells may play an important role in its mechanism.