Nucleotide variation in the triplet codon coding for amino acid position 72 of human serum amyloid A1 (SAA1), which was suggested by amino acid sequence analysis, was analyzed here in order to identify the genomic sequences coding SAA1.2 and to characterize further the SAA1 allele frequency in a Japanese population. The SAA1 exon 4 was amplified by PCR and treated with Nco I. Sequencing of PCR products from genomic DNA of individuals who were heterozygous for the Nco I site revealed GGT (Gly) and GAT (Asp) at the position 72. The allele having 72Asp showed the exon 3 polymorphism coding 52Ala and 57Val. This allele should thus be identified as SAA1.2. Alleles with 72Gly were either 52Val and 57Ala(SAA1.1) or 52Ala and 57Ala (SAA1.3) or 52Ala and 57Val (SAA1.5). The frequency of SAA1 alleles in the 321 Japanese subjects was 0.310, 0.012, 0.347 and 0.330 for each SAA allel of 1.1, 1.2, 1.3 and 1.5, respectively. The presence of the SAA.4 allele was not evaluated.