Background Vitamin D3 and its metabolites have been found to exert immunosuppressive effects both in vivo and in vitro. We investigated the synergistic effect of calcitriol and cyclosporine A (CsA) on lymphocyte proliferation in vitro and graft rejection following rat liver allotransplantations in vivo.Methods Alloantigen driven, human peripheral mononuclear cells proliferation and cytokine production capacity were tested in the presence or absence of various concentrations of calcitriol or CsA. In vivo, liver allografts were transplanted in a high responder strain combination (SD to Wistar) rats and combination of subtherapeutical dose of CsA and calcitriol was administered in recipients, whereas the control recipients received single or no immunosuppressant. Proliferation of splenocyte from recipient was tested with mixed lymphocyte reaction. Serum interleukin-2 (IL-2) and interferon gamma (IFN-γ) concentrations were measured with enzyme linked immunosorbent assay.Results Combined medication of 10-9 mol/L calcitriol and 100 ng/ml CsA inhibited human peripheral mononuclear cells proliferation to alloantigen and the production of IL-2 and IFN-γ but promoted that of IL-4and IL-10. Similarly, combination of 250 ng· kg-1 · d-1 calcitriol and 1.0 mg · kg-1 · d-1 CsA showed an additive effect in liver transplant model. It restrained splenocyte proliferation to alloantigen from donor and significantly reduced serum concentration of IL-2 and IFN-γ in recipients. Consequently, allograft rejection in combined medication group was minor (median Williams grade was 1.0 vs 3.0 in combined medication group and in the control group, P＜0.05) and the recipients survival was evidently prolonged [(93.7±5.8) days vs (12.6±1.4) days in combined medication group and in the control group, P＜0.01].Conclusion A combination of calcitriol and CsA has an additive effect on limiting lymphocyte proliferation and prolonging liver graft survival. With its additional immunomodulating property, calcitriol is a potent immunosuppressant that can extend the therapeutic window of classical immunomodulators in prevention and treatment of liver graft rejection.