Insulin-dependent diabetes mellitus (IDDM) is characterized by a progressive autoimmune destruction of pancreatic beta cells. Many data suggest that interleukin 1 (IL-1) plays a fundamental role in the pathogenesis of the disease. In the nonobese diabetic (NOD) mouse, a spontaneous model of IDDM, it was put forward that the disease is linked to a susceptibility locus, called idd5, which contains the IL-1 receptor (IL-1R) gene. The polymerase chain reaction (PCR) was used to characterize the IL-1R gene in our NOD mouse colony and in two mouse strains taken as controls. Using primers to amplify the IL-1R gene between bp-106 and +378, a 580 bp fragment was obtained from C57BL/6 DNA but not from DBA/2 and NOD DNA. However, amplification of the IL-1R gene region between bp +1 and +378 in the three strains yielded amplicons 480 bp long. The specificity of the amplification was confirmed by restriction analysis. Our results suggest, depending on the strain, the presence of one or two introns: one (480 bp) in the 5'-untranslated region and the other (100 bp) in the region coding for amino acids between 69 and 126, and an exon-intron organization of the mouse IL-1R gene different than that described in the human genome.