A novel method, based upon primer extension, has been developed for measuring the reopening temperature of a single type of DNA hairpin structure. Two DNA oligo-nucleotides have been utilized and designated as primers 1 and3¢-termini fully comple-mentary to the hairpin flanking sequences, was used to evaluate primer extension conditions, and primer 2, with its 3¢-end competing with the DNA hairpin stem, was used to detect the DNA hairpin reopening temperature.