Objective To explore prevention of cyclosporine A (CsA) combined with Cobalt protoporphyrin (CoPP) against murine graft versus-host disease (GVHD) after allogeneic hematopoietic stem cell transplantation (allo-HSCT).Methods C57BL/6 (H-2Kb) mice were used as donors and BALB/c (H-2Kd) mice as recipients,which were randomly divided into 4 groups.The mice in total body irradiation group (TBI group) were lethally irradiated and injected intravenously with PBS; The mice in Allo-HSCT group (BS group) were lethally irradiated and injected intravenously with bone marrow cells and spleen cells; The mice in CsA intervention group (CsA group) were injected with CsA intraperitoneally after allo-HSCT; The mice in CsA combine with CoPP intervention group (combination group) received both CsA and CoPP intraperitoneally after alloHSCT.Recipients were monitored for condition,survival rate and weight.The liver,small intestine and skin in the recipients were gained and pathological changes of GVHD were assessed.The kidney was stained with Masson staining dye to observe the tissue fibrosis.The expression levels of renal HO-1 mRNA in the recipients were detected.Results In contrast to BS and CsA groups,GVHD degree in combination group was mild,with less reduction and quick recovery of weight.On the day 30 after HSCT,survival rate in BS group was 36.8％,and that in combination group and CsA group was 69.6％ and 53.5％ respectively (P＜0.05).In comparison with BS and CsA groups,pathological changes in combination group were mild,cellular edema and degeneration degree of the liver,small intestine and skin were slight,and few necrosis and infiltrated inflammatory cells were observed.Tubulointerstitial fibrosis hardly occurred in combination group,but it occurred in CsA group abundantly.As compared with BS group,the expression levels of HO-1 mRNA was increased in combination group,while decreased in CsA group (P＜0.05).Conclusion CsA combined with CoPP enhanced the protective effect of CsA against GVHD,moreover,CoPP could alleviate the side effects of CsA,which might be related with up-regulation of the expression levels of HO-1.