The characterisation of the metabolic fate of a new drug molecule is essential in the course of the drug discovery and development process. As a consequence, potential human drug metabolites must be available in amounts of at least a few milligrams of pure compound. Metabolite preparation can become a significant time and cost factor, if elaborate multi-step chemical syntheses become necessary. Dr techn. Margit Winkler at ACIB, Austrian Centre of Industrial Biotechnology, answers this question: how can we utilise microbes to make authentic human metabolites?