The seasonal variation in the chemical composition of the leaf essential oil of Liriodendron tulipifera has been analyzed by GC-MS. Two individual trees were sampled five times during the course of the growing season. Twenty components were identified in the leaf oils, which were dominated by sesquiterpene hydrocarbons, principally germacrene D and beta-elemene, in the early part of the season (42-44% and 18-23%, respectively,) but monoterpene hydrocarbons, largely (Z)-beta-ocimene, dominated the later season leaf oils (40-60%). The leaf oils exhibited in-vitro antibacterial activity against Bacillus cereus and Staphylococcus aureus as well as cytotoxic activity on MDA-MB-231 and Hs 578T human breast tumor cells.