The polymerase chain reaction catalyzed by Taq DNA polymerase has been used for site-specific mutagenesis. The amplification was primed by two oligodeoxyribonucleotides complementary to insulin receptor cDNA. To direct the synthesis of mutant DNA, mismatches were introduced into one of the primers. Six different mutations were constructed by this technique. Of twelve clones whose sequences were determined, ten (83%) had the correct sequence. This technique, which does not require the use of single-stranded DNA templates, provides a simple and efficient approach to site-specific mutagenesis.