Pseudolaric acid B (PB) is a major bioactive component extracted from Pseudolarix kaempferi. Traditional medicine practitioners in China have been using the roots of this plant to treat inflammatory and microbial dermatoses for centuries. In this study, the experiments were conducted to investigate immunosuppressive activity of PB on T lymphocytes in vitro. The results show that PB has hypotoxicity on hepatocytes and renal cells assessed by Methyl thiazolyl tetrazoliym (MTT) Assay and selectively inhibits the proliferation of T lymphocytes and antigen-specific T lymphocytes stimulated by concanavalin A and dinitrophenyl-modified spleen cells, respectively. Then, PB also shows suppressive effect on T lymphocytes in mixed lymphocyte reaction. Moreover, PB can depress the production of interleukin-2 (IL-2) secreted by concanavalin A-activated T lymphocytes or dinitrophenyl-modified spleen cells-activated antigen-specific T lymphocytes. It is first found that PB exhibits the inhibitory action on antigen-specific T lymphocytes and on the production of IL-2 from antigen-specific T lymphocytes and selectively inhibits the proliferation of T lymphocytes and the production of IL-2 as well. PB is in need of further study because of its potent immunosuppressive function.