A rapid method for the saynthesis of oligodeoxynucleotides (ODNs) terminated by 5 '-amino-5 ' -deoxythymidine is described. A 3 '-phosphorylated ODN (the donor) is incubated in aqueous solution with 5 '-amino-5 ' -deoxythymidine in the present of N-(3-dimethyl-aminopropyl)- N-ethylcarbodiimide hydrochloride (EDC), extending the donor by one residue via a phosphoramidatre bond. Template- directed ligation of the extended donor and an acceptor ODN, followed by acid hydrolysis, yields the acceptor ODN extended by a single 5 ' -amino-5 ' -deoxythymidine residue at its 5 ' terminus.