The crude ethanol extract from the leaves of Dendropanax cf. querceti (Araliaceae) from Monteverde, Costa Rica, exhibits cytotoxic activity against Hep-G2, A-431, and H-411E tumor cell lines. The active component has been isolated by activity-directed separation and identified by 1H-and 13C-NMR spectroscopy as the triterpene lupeol. The mechanism of cytotoxic activity of lupeol has been determined to be inhibition of topoisomerase II.The Araliaceae family contains about 65 genera and more than 800 species, primarily tropical (1, 2), especially in the lndo-Malayan region and tropical America. Although members of the Araliaceae in North America, Europe, and Asia have been studied extensively and have been seen to contain a profusion of substances with marked bioactivity, there are, on the other hand, abundant woody members of the family found in the Neotropics which have been poorly studied or completely neglected. We have been interested in examining the phytochemistry and biological activity of members of the Araliaceae from the montane cloud forest of Monteverde, Costa Rica (3). Dendropanax querceti J. D. Smith in its typical form is a shade tolerant understory to small lower canopy tree, which grows in Costa Rica in the montane rain forests of the Cordillera de Talamanca at 2200-2700 m (4). Individuals from the smaller Cordillera de Tilaran grow at lower elevation (1000-1800m) in the premontane and lower montane rain forests on the Caribbean slope and crest of the range (5). Trees from the Cordillera de Tilaran differ from typical D. querceti in leaf size and shape, and may represent an undescribed species (W. Haber, personal communication). Hence we refer to the population of the Cordillera de Tilaran as Dendropanax cf. querceti. In this paper we report the isolation, purification, and activity of the cytotoxic component from leaves of Dendropanax cf. querceti. D. querceti has, to our knowledge, no current economic utility.Leaves of D. cf. querceti were collected during June, 1991, from trees growing in lower montane moist forest at ca. 1400m a.s.l. at Monteverde, Costa Rica, and were immediately extracted using a Soxhlet extractor and refluxing 95 % ethanol.Identity of the plants was determined (R. 0. Lawton) by comparison with the collections of the Museo Nacional de Costa Rica, the Field Museum, and the Missouri Botanical Garden. Vouchers have been deposited with the University of Alabama in Huntsville and the Museo Nacional de Costa Rica. The fresh leaves (120g) were cleaned of epiphylls and other debris, were chopped and were then extracted with refluxing ethanol for three hours. The crude extract was assayed for cytotoxic activity and exhibited remarkable cytotoxicity (93 % inhibition at a concentration of 0.02 5 % w/v) against human Hep G2 hepatocellular carcinoma cells (3). The cytotoxicity of the crude extract on tumor derived cell lines was measured using the MTT assay (6) for cell viability. As shown in Table 1, at a concentration of 0.02 5 % w/v, the crude extract of D. cf. querceti showed 93 % kil...