Oridonin (1), lasiokaurin (2), enmein (8), and enmein-3-acetate (9) and related compounds (3 and 10, ) all of which have α-methylene cyclopentanone function in their molecule, have been shown to have antitumor activity against Ehrlich ascites carcinoma inoculated into mice. These compounds have also indicated specific activity against gram-positive bacteria. On the other hand, oridonin dihydro-derivative (4), compound (5), trichokaurin (6), and dihydroenmein (11) show any activity against neither tumor nor bacteria. Thus, it is concluded that the α-methylene-cyclopentanone system must be an important active center. Biomimetic reactions of oridonin and enmein with several thiols etc. support this conclusion.