About 50% of Japanese have been estimated to possess at least one ALDH2*2 allele with a substitution of AAA for GAA at codon 487 of the aldehyde dehydrogenase gene. This mutation is tightly associated with the sensitivity of an individual to alcohol. We developed a method of identifying the ALDH2*2 allele by a non-radioactive technique. DNA from individuals was subjected to polymerase chain reaction in which part of the aldehyde dehydrogenase 2 gene was amplified. After dot-blotting onto nylon membranes, the DNA was hybridized with biotin-labelled allele-specific oligonucleotides. Determination of genotypes on 77 unrelated healthy Japanese individuals, using the conventional method and our new method with gradient hybridization temperature and competitive oligonucleotides, indicated that the latter was superior to the former.