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The three-dimensional structure of bilayer-associated gramicidin A is available from a structural data base. This and related peptides are, therefore, ideal model compounds to use during the implementation and development of new NMR techniques for the structural investigations of membrane proteins. As these methods rely on the isotopic labelling of single, selected or all sites, we have, investigated and optimised biochemical protocols using different strains of the Gram-positive bacterium Bacillus brevis. With newly developed schemes for isotopic labelling large amounts of gramicidin and tyrocidin enriched with stable isotopes such as 15N or 15N/13C have been obtained at low cost. A variety of analytical and spectroscopic techniques, including HPLC, mass spectrometry and NMR spectroscopy are used to characterise the resulting products.