Point mutations involving codons 12, 13, and 61 of the N-ras gene are found in patients with acute myeloid leukemia (AML). We have developed a sensitive assay for the analysis of these mutations which we have called allele-specific enrichment. In this protocol the polymerase chain reaction (PCR) amplifies DNA with primers that introduce new restriction sites into the normal N-ras allele only. Digestion with the appropriate enzyme cleaves normal, but not mutant, alleles and this digested product provides a mutant allele-enriched template for a second round of amplification. The second PCR product is digested, Southern blotted and analyzed by allele-specific oligonucleotide (ASO) hybridization. This protocol is more sensitive than ASO hybridization alone and has revealed a minor clone in the DNA of a patient with AML. The method may be useful for the detection of minimal residual disease in a subset of patients in remission.