Nine lignans and two butanolides were isolated from the stem bark of Machilus thunbergii and their structures were identified as machilin A (1), licarin B (2), zuonin B (3), macelignan (4), secoisolancifolide (5), isolancifolide (6), oleiferin C (7), meso-dihydroguaiaretic acid (8), licarin A (9), machilin F (10), and nectandrin B (11) by spectroscopic means. These compounds were assessed for their abilities to activate a caspase-3 activity in human promyeloid leukemic HL-60 cells. The intracellular caspase-3 activity of macelignan (4), oleiferin C (7), meso-dihydroguaiaretic acid (8), and licarin A (9) increased approximately 3.04, 6.16, 2.10, and 3.10-fold at 100 microM over that of untreated control. In addition, compounds 4, 7, 8, and 9 induced internucleosomal DNA fragmentation in HL-60 cells.