A simple and easy method for stabilizing DNA triptexes using Ag+ is reported. A silver ion displaces the N3 proton of cytosine in Hoogsteen base-pairing to form the base triplet CG.CAg+. By the addition of an equimolar amount of Ag+, the third-strand 15-mer sequence containing five cytosines was stabilized by similar to 30 degrees C in melting temperature at pH 7. The triplex structure was stable even under weakly basic conditions.