Although several plant-derived drug groups (vinca alkaloids, taxanes, podophyllotoxin derivatives and camptothecins) continue to be widely used in cancer therapy, the anticancer potential of the Plant Kingdom remains largely unexplored. In this work, we have carried out a random screening for selective anticancer activity of 57 extracts from 45 plants collected in Grazalema Natural Park, an area in the South of Spain of high plant diversity and endemism. Using lung cancer cells (A549) and lung non-malignant cells (MRC-5), we found that several extracts were more cytotoxic and selective against the cancer cell line than the standard anticancer agent cisplatin. Five active extracts were further tested in cancer and normal cell lines from other tissues, including three skin cell lines with increasing degree of malignancy. An extract from the leaves of Daphne laureola L. (Thymelaeaceae) showed a striking potency and selectivity on lung cancer cells and leukemia cells; the IC₅₀ values against these cancer cells were approximately 10000-fold lower than against the normal cells. Daphnane-type diterpene orthoesters may be responsible for this highly selective anticancer activity.