An efficient method has been developed to generate base substitution mutations within deletion loops of DNA heteroduplexes. This method utilizes a heteroduplex formed between a deletion mutant cloned in a plasmid vector and its wild-type counterpart from which two restriction sites had been removed from the vector. The heteroduplex is exposed to sodium bisulfite to deaminate cytosine residues in the single-stranded loop, and the mutagenized plasmid DNA is used to transform a strain of bacteria lacking the enzyme uracil N-glycosylase. Pooled progeny DNA is digested with the two restriction enzymes, whose sites had been mutated in the wild-type plasmid, to eliminate the original deletion mutant DNA. Point mutants with C . G-to-T . A transitions are obtained at high frequency after a second transformation. To test the feasibility of the approach, the tetracycline resistance gene of pBR322 was chosen as the target sequence. It was found that the proportion of tetracycline-sensitive transformants increased as both the size of the heteroduplex loop and the time of incubation with the mutagen increased and this varied from 20% up to 70%. Nucleotide sequence analysis of several tetracycline-sensitive mutants confirmed that C-to-T transitions had been produced in the segment of DNA corresponding to the deletion loop.