A method to introduce multiple mutations and to reconstruct genes, using a single oligodeoxyribonucleotide and DNA polymerase with high processivity, such as modified T7 DNA polymerase [Tabor and Richardson, Proc. Natl. Acad. Sci. USA 84 (1987a) 4767-4771], is described. A eukaryotic cDNA, coding for porcine growth hormone (pGH), was reconstructed in this study to delete 75 bp and to introduce a G----A transition. The deletion removes 75 bp and brings an ATG just upstream from the codon for the first amino acid in the mature protein. Moreover, the G----A substitution creates a new PvuII restriction site to facilitate further manipulation of the gene. Maximum mutation frequency with this multisite-directed mutagenesis is reached within 15 min with an efficiency approaching 50%, when using the modified T7 DNA polymerase. No multisite-directed mutants were obtained when T4 DNA polymerase or Klenow (large) fragment of DNA polymerase I were used. The described method is also applicable to simple single site-directed mutations as well as to more complex gene reconstruction strategies.