This article considers the application of topical analytical techniques-electrospray ionisation-mass spectrometry (ESI-MS) and liquid chromatography (LC)-ESI-MS - to the characterisation of selected small molecular-mass synthetic and natural-product pharmaceuticals, as well as bioactive peptides. We chose the synthetic drugs according to selected structural classes in which they give ESI signals primarily as [M + H](+) ions. The structural classes chosen to illustrate the application of ESI-MS and LC-ESI-MS to such drug characterisation are drugs with amine-containing side chains, drugs with N-containing saturated ring structures, and 1,4-benzodiazepines and other heterocyclic hypnotics. We then discuss potential natural-product pharmaceuticals, such as selected quinolines and nicotines. We next apply the technique to the characterisation of molecules of unknown structure present in plant extracts or fractions that possess antibacterial activity. Finally, we discuss the isolation and the characterisation of bioactive peptides from frog-skin secretions utilising an arsenal of MS tools as well as Edman degradation sequencing and cDNA cloning. (c) 2006 Elsevier Ltd. All rights reserved.