The formation of isodesmosine and desmosine in vitro by the action of lysyl oxidase on tropoelastin was studied. The synthesis of desmosines occurred in the absence of additional substances. The formation of desmosines was not affected by removal of molecular O2 from the reaction medium nor was it affected by the lack of proline hydroxylation in tropoelastin. However, there was virtually no desmosine formation at 15 degrees C, a temperature not conducive to coacervation, indicating that coacervation is an important prerequisite for cross-linking.