Norcantharidin (NCTD) is a synthetic analog of cantharidin. It has potent antitumor properties without obvious side effect that Cantharidin has on urinary organs. It has been reported that NCTD is an immunological stimulator to NK, LAK, neutrophil and lymphocyte. In our experiment, however, we observed that NCTD can markedly inhibit lymphocyte proliferation stimulated by mitogen ConA or LPS in vitro, and the mixed lymphocyte reaction (MLR) of mice, in a dose-related manner. This occurred even when the drug was added 40 hours (for lymphocyte proliferation) or 72 hours (for MLR) after the cultures were initiated. On the other hand, NCTD has no effect on inactive lymphocytes that were cultured as control in medium without mitogens, suggesting that NCTD selectively acts on activated lymphocyte.