Distinct effects of Oridonin and Lasiodonin, which were extracted from Isodon serra was compared by the ratio of IC(50) versus EC(50), the therapeutic index. After choosing the more effective one, Oridonin, its immunosuppressive effect and mechanism were investigated using BALB/c mouse splenic lymphocytes both in vitro and in vivo. When murine splenic lymphocytes was incubated with Oridonin, the novel extract effectively suppress the overproduction of the cell stimulated by Concanavalin A in a dose and time-dependent manner. This inhibitive activity was mainly due to interfering DNA replication in G(1) stages and regulating cell cycle and minorly due to decreasing the CD4(+)/CD8(+) lymphocytes level, according to Flow cytometry analyses (FCAS) results. Xylene-induced mouse tumescence model result suggested that Oridonin depressed the murine ear-swelling extent and the level of Interleukin-2 in the blood serum of experimental animals. The exciting results of enzyme-linked immunosorbent assay (ELISA) indicated that Oridonin could inhibit the secretion of Interleukin-2, Interferon-gamma, Interleukin-12p40 and Tumor necrosis factor-alpha in murine splenic lymphocytes. Moreover, the results of reverse transcriptase-polymerase chain reaction (RT-PCR) confirmed that the inhibition was through decreasing the expression level of these cytokines mRNA. Consequently, the results of our research showed that Oridonin suppressed overproduction of murine splenic lymphocytes through interference of DNA replication, regulation of cell cycle and inhibition of cytokine secretion both at protein and mRNA level.