The allelic forms of the HLA-DQB gene have been recognized as susceptibility markers of type 1 diabetes mellitus. One of these alleles, the DQw3.2 (DQw8), accounts for the well-documented association of the DQw3 locus with the disease. This report describes a method using the polymerase chain reaction mismatch technique to amplify the three different DQw3 allele sequences in 26 insulin-dependent diabetic patients. Primers were designed that differed only at one base at the growing end of their sequences. Using a common oligonucleotide primer located downstream in the first domain of the DQB gene and three other primers located at the other end of the sequence being amplified, it was possible to identify and distinguish the DQw8 allele from the other two closely related alleles (DQw7, DQw9). This method, which could be useful in excluding HLA-related susceptibility to diabetes mellitus, is rapid and nonisotopic, and indeed could be adapted to investigate any DNA sequence polymorphism.