M13 RF IV DNA where phosphorothioate groups are incorporated at restriction endonuclease Nci I recognition sites in the (-)strand is efficiently nicked by the action of this enzyme. Incubation of such nicked DNA with exonuclease III produces gapped DNA. The gap can be filled by reaction with deoxynucleoside triphosphates and DNA polymerase I. When this sequence of reactions is performed with DNA containing a mismatch oligonucleotide primer in the (-)-strand mutational frequencies of 70-90% can be obtained upon transformation. The general nature of this methodology has been further shown to be applicable to other restriction enzymes such as Hind II, Pst I and Fsp I. The mutational frequency obtained using these enzymes is between 40-80% mainly because of less efficient nicking and gapping. Studies on inhibition of Nci I cleavage show that in addition to a phosphorothioate group at the position of cleavage an additional group in the 5'-neighbouring position is necessary for complete inhibition.