We investigated whether a G123-->A mutation causing a Gly40-->Ser substitution in exon 2 of the human glucagon receptor gene, which has been reported to be associated with non-insulin-dependent diabetes mellitus (NIDDM) and impaired glucose tolerance (IGT) in France and Sardinia with a prevalences as high as 4.6% and 8.3%, respectively, is associated with Japanese patients with glucose intolerance. This mutation was not found in 242 unrelated Japanese patients with NIDDM or 23 with IGT by screening by the polymerase chain reaction-restriction fragment length polymorphism method. We also performed single-stranded conformational polymorphism analysis to search for new mutations in this gene associated with glucose intolerance. We found no mutations by examining all the 13 exons from 30 selected patients with NIDDM who had at least 2 diabetic first-degree relatives. These patients were also screened for the common polymorphism at codon 155 reported previously, but none were found to carry it. The absence of the mutation and polymorphism, which are common in French and Sardinian NIDDM or IGT patients, in Japanese indicates the existence of marked ethnic differences.