With the use of an enzymatic replacement method, 90%-enriched [1-13C]lysine was introduced into the reactive site of the basic pancreatic trypsin inhibitor. Characterization of the labelled inhibitor with 13C nuclear magnetic resonance (NMR), 1H NMR and chemical methods showed that while the reactive-site peptide bond Lys-15--Ala-16 was properly resynthesized, the polypeptide chain was cleaved at the peptide bond Arg-39--Ala-40 and Arg-39 was removed. Detailed 1H NMR studies showed further that, with the exception of the immediate environment of the modification site, the average spatial structure of the native inhibitor was preserved in the modified protein. Compared to the native inhibitor, the thermal stability of the globular conformation was found to be reduced, interior amide protons exchanged at a faster rate and the internal mobility of aromatic rings located outside the immediate environment of the cleaved peptide bond was essentially unchanged. These observations coincide closely with previous reports on different modifications of the inhibitor and can be explained by a recently proposed dynamic multi-state model for globular proteins. Since the fundamental structural properties of the native inhibitor and full inhibitory activity are preserved after resynthesis, the [1-13C]lys-15-labelled inhibitor with the peptide bond Arg-39--Ala-40 cleaved and Arg-39 removed should be suitable for 13C NMR studies of mechanistic aspects of proteinase-inhibitor interactions.