The pentane and CHCl3 fractions of a crude extract of Michelia floribunda exhibited cytotoxic activity when tested in KB and P388 tumor cell cultures. Repeated chromatography led to the isolation of three cytotoxic sesquiterpene lactones (costunolide, parthenolide, and santamarine) and a cytotoxic isoquinoline alkaloid (liriodenine). Inactive sesquiterpene lactones obtained during the course of this study included dihydroparthenolide and two new glucosides of dihydrotamaulipin A and dihydroreynosin (1 and 2). The structures of these new compounds were determined through interpretation of their spectroscopic data including 2D-NMR spectroscopy. Syringin was also isolated from the extract.