We have investigated restriction fragment length polymorphism (RFLP) of HLA and non-HLA regions of the genome in the homogeneous Danish population. Insulin-dependent diabetes mellitus (IDDM) patients and healthy individuals were selected for being HLA-DR 3/4 heterozygous to evaluate the influence of genes other than DR on disease susceptibility. Five different probes were used: HLA alpha and beta DQ (chromosome 6), the Ins 310 genomic fragment which detects a polymorphic region 5' to the insulin gene (chromosome 11), and cDNA for the constant regions of the T cell receptor alpha and beta genes (chromosomes 14 and 7). Fifteen cells homozygous for the HLA-D region were used to obtain reference DNA patterns. This allowed us to describe four splits among the HLA-DQw3 haplotypes (DQw3.1 to DQw3.4). The two new haplotypes DQw3.3 and DQw3.4 do not code for the TA10 serological marker which is found on DQw3.1 positive cells. One-hundred per cent of IDDM patients were typed as DQw3.2 versus 68 per cent for controls (p = 0.003). However, our results do not indicate a role for the Ins 310 or for the alpha DX locus region in IDDM susceptibility, in contrast to previous reports by others. The restriction enzymes that we have used did not reveal significant differences between DNA patterns of patients and controls with probes for the constant region of the T cell receptor genes.