The cytokine interleukin (IL)-12 is bound by a heterodimeric receptor and mediates a range of immunological activities, in particular, favouring the development of uncommitted T cells to the Th1 phenotype. Genes encoding elements of the IL-12 pathway are therefore good candidates for mediating susceptibility or resistance to a range of immune disorders, including Type I diabetes. We made a systematic search for variants in the human gene encoding the low-affinity IL-12 receptor, IL12RB1. Four single-nucleotide polymorphisms and two microsatellite polymorphisms were defined. We also tested these IL12RB1 alleles for involvement in Type I diabetes susceptibility, testing 131 families. Although suggestive evidence for linkage to a susceptibility gene was found, none of the IL12RB1 variants we defined demonstrated preferential transmission in these families.