Phosphorylated FTY720 is an analog of Sphingosine 1 Phosphate (S1P) with immunosuppressive activity that negatively regulates the expression of S1P-Receptor 1. It also inhibits the migration of CD4 and CD8 single-positive T cells from the thymus to the periphery, sequesters peripheral blood lymphocytes in lymph nodes and Peyer's patches, and delays the exit of effector T cells toward the graft. The aim of our work was to study the effect of FTY720 on the kinetics of skin allograft rejection in a fully mismatched model; euthymic (Euthy) versus thymectomized (ATX) C57BL/6 mice (haplotype H-2(b)) recipients of BALB/c mice (haplotype H-2(d)) donor cells. The animals were injected daily with FTY720 (1 mg/kg) intraperitoneally for 2 weeks. To monitor the humoral immune response, serum samples collected at day 0 (pre-immune) and at day 23 after skin graft rejection were examined using BALB/c thymocytes as antigens in flow cytometry. To confirm the effect of FTY720 on peripheral lymphocytes, peripheral blood was analyzed by flow cytometry. Euthy and ATX FTY720-treated mice showed prolongation of skin allograft survival when compared with nontreated Euthy and ATX controls (P < .005). Unexpectedly, FTY720-treated Euthy mice showed significantly delayed graft rejection when compared to similarly treated ATX mice (P < .005). The delayed graft rejection in FTY720-treated Euthy mice correlated with a reduced content of Th1-mediated IgG(2a) and IgG(2b) antibodies when compared with FTY720-treated ATX mice (P < .05). In conclusion, FTY720 delays the kinetics of allograft rejection in a fully mismatched model by inhibiting Th1-mediated humoral immune responses. The presence of the host thymus appears to be required for this phenomenon.