Gas chromatography (GC) can be used to characterize microorganisms chemically. Microbial whole cells and cell extracts can be analysed by GC after being thermally or chemically degraded, i.e. pyrolysed or hydrolysed. Microbial metabolic products can be detected by GC analysis of culture media or the atmosphere above such media, i.e. headspace analysis. Pyrolysis products can be analysed direct, while hydrolysis products and non-volatile metabolites must be derivatized before being chromatographed. Differences in the chromatographic patterns can be used to differentiate between microorganisms. Detection of metabolic products in body fluids by means of GC constitutes a potential technique for the rapid diagnosis of infectious diseases.