TW19 is a diterpene compound isolated from the root cortex of Tripterygium Wil- fordii Hood. F.. Its effects on the immune function in ICR mice and the male fertility in SD rats, ICR mice and Kunming mice were evaluated. TW19 was given orally for 5 weeks. Then the antifertilitic effect was assessed by mating test. The results showed that ED50, for anti-fertilitic were 332μg/kg, 369. 9μg/kg and 286. 8 μg/kg per day in SD rats, ICR mice and Kunming mice respectively. After the treatment of TW19 at antifertility dose for 5 weeks consecutively, the spermatozoa density and motility of trial animals reduced significantly, The weight of testic also declined in SD rats and Kunming mice, but no effects were observed in ICR mice. TW19 inhibited prolifera- tion of splenic T and B-lymphocytes of ICR mice in vitro and hence inhibited the anti- body formation in vivo, but compared with T 4, the immunosuppressive effect of TW19 was less obvious. TW19 is a diterpene compound isolated from the root cortex of Tripterygium Wil- fordii Hood. F .. Its effects on the immune function in ICR mice and the male fertility in SD rats, ICR mice and Kunming mice were evaluated. TW19 was given orally for 5 weeks. Then the antifertilitic effect was assessed by mating test. The results showed that ED50, for anti-fertilitic were 332 μg / kg, 369. 9 μg / kg and 286. 8 μg / kg per day in SD rats, ICR mice and Kunming After the treatment of TW19 at antifertility dose for 5 weeks consecutively, the spermatozoa density and motility of trial animals reduced significantly, The weight of testic also declined in SD rats and Kunming mice, but no effects were observed in ICR mice. TW19 regulated prolifera- tion of splenic T and B-lymphocytes of ICR mice in vitro and therefore inhibited the anti- body formation in vivo, but compared with T 4, the immunosuppressive effect of TW19 was less obvious.