A new neolignan, (7R,8R)-5-O-demethylbilagrewin (1), together with four known lignans (2-5), were isolated from the heartwood of Santalum album (Santalaceae). The structure of 1 was determined by analysis of extensive spectroscopic data. The isolated compounds and derivatives were evaluated for their cytotoxic activities against HL-60 human promyelocytic leukemia cells and A549 human lung adenocarcinoma cells. Compounds 1 and 2 exhibited cytotoxicity against HL-60 cells with IC(50) values of 1.5+/-0.02 and 4.3+/-0.13 microM, and against A549 cells with IC(50) values of 13.6+/-0.32 and 19.9+/-1.27 microM, respectively. The aldehyde group of 1 and 2 was revealed to be a structural requirement for the appearance of cytotoxicity in this type of lignans. These tumor cell deaths were shown to be mediated through induction of apoptosis.