The emergence of drug-resistant microbes left us with a great need for new antimicrobial agents. Flavonoids, with their wide range of biological activities, are good candidates in this respect. Although naturally occurring flavonoids are the most studied ones, semi-synthetic or synthetic flavonoids have proven to have great potential, inhibiting and even killing microbes at concentrations below 1 mu g ml(-1). The substitution pattern of these flavonoids often includes hydroxy groups, halogens or other heteroatomic rings, such as pyridine, piperidine or 1,3-dithiolium cations. However, the great variety in substituents makes it difficult to draw any definitive conclusion regarding their structure-activity relationship.