A convenient method for the synthesis of l-tryptophan analogues is described. The method utilizes E. coli tryptophan synthetase, which catalyses the condensation of indole and l-serine to yield l-tryptophan. It is found that several indole analogues will replace indole as substrate for the enzyme to give the corresponding l-tryptophan analogues in good yield. By using [ 14C]serine, analogues can be prepared radioactively labeled in the side-chain carbon atoms.