Gas chromatography of the component fatty acids with added internal standard can be used to determine a wide variety of neutral and polar lipids, after separation by thin-layer chromatography, if precautions are taken to minimise losses. Results can be expressed in terms of the relative amounts of fatty acids contained by the lipids or the actual weights and molar amounts of the lipid classes can be calculated by applying readily derived factors. The procedure is at least as accurate as others in current use and has the additional merit of allowing simultaneous determination of lipid and fatty acid composition.