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Plants have a rich history as medicines for human use. Recently introduced plant-derived drugs include paclitaxel from Taxus (Taxaceae) species for the treatment of cancers, artemisinin for malaria therapy and the alkaloid galanthamine for the management of Alzheimer's disease. A judicious combination of chemical and biological screening techniques can provide a steady supply of lead compounds for the improved treatment of human ailments. By this means, candidate compounds can be rapidly selected for further development, as is the case for xanthones from gentians and coumarins from Peucedanum ostruthium (Apiaceae), which may be of relevance in Alzheimer's disease. Encouraging results are also being obtained for hypericin from Hypericum perforatum (Hypericaceae) in the photodynamic therapy of cancer. An aspect that must not be forgotten is that the usual biological screening procedures do not pick up prodrugs which have to be metabolized by the human body into active analogues. A good example is salicin from Salix species (Salicaceae) which is metabolized in the body to give the anti-inflammatory and analgesic compound salicylic acid. Investigations which include simulation of metabolism are thus of primordial importance.

Plants represent an extraordinary reservoir of novel molecules and there is currently a resurgence of interest in the vegetable kingdom as a provider of new lead compounds for introduction into therapeutic screening programs. Plant constituents of interest are usually isolated following a bioactivity-guided fractionation procedure. In order to render this approach more rapid and efficient, the dereplication of crude plant extracts with LC-hyphenated techniques (LC/UV-DAD, LC/MS and LC/NMR) represents a strategic element to avoid finding known constituents and to target the isolation of new bioactive compounds. The development of simple and rapid bioassays is also crucial for an efficient localisation of active principles. In this respect, the use of bioautographic methods, which enable a direct screening of bioactive constituents on TLC is of great interest. The combination of chemical and biological screening has proved to be very efficient for the survey of an important number of extracts and for the discovery of new lead compounds and several examples of this approach will be discussed in this paper.