FTY720 is a novel immunosuppressant that is highly effective in inhibiting rejection of allografts and autoimmunity in various animal models. It has been shown that the sphingosine 1 phosphate (S1P) receptors are the direct molecular targets of FTY720. However, the mechanisms responsible for inhibiting specific immune responses by FTY720 are not well resolved. In particular, there is no available information on whether or how this compound affects humoral immunity. We have investigated the effect of FTY720 treatment on B-cell response during the immune response to a well-defined T-dependent antigen. Our data demonstrated that germinal center reaction was significantly reduced in peripheral lymphoid tissues of mice treated with FTY720. In addition, FTY720 treatment inhibited the production of high-affinity, class-switched antibodies, but not the production of low-affinity, immunoglobulin M (IgM) antibody. Consistently, FTY720 did not have a significant effect on antibody response to a T-independent antigen. Our results may have important implications in application of FTY720 in immune regulation.