Twenty Black families in which at least two siblings had non-insulin-dependent diabetes mellitus (NIDDM) were typed for restriction-fragment-length polymorphisms at the insulin (INS), insulin-receptor (INSR), and HLA-DR beta loci. Evidence for linkage between NIDDM and these loci was assessed with various genetic models for the transmission of NIDDM and with the affected-sib-pair approach, which does not require assumptions concerning a genetic model for NIDDM. Tight linkage between NIDDM and any of the loci was unlikely under all of the genetic models examined. Similarly, for all three of the loci, the distribution of affected sib pairs sharing 2, 1, or 0 genes identical by descent was not significantly different from (and was very similar to) that expected if the locus were unrelated to disease susceptibility. There was no evidence for linkage heterogeneity for any of the loci when families were grouped according to obesity or age at onset or when considering families individually. We conclude that the INS and INSR loci can be ruled out as major susceptibility loci for NIDDM in most Black families segregating this disorder, but we recognize that defects at either of these loci may cause or contribute to NIDDM in some patients. In addition, it is possible that variation at the INS and/or INSR loci may contribute to NIDDM susceptibility by modifying susceptibility due primarily to another major gene(s) or as part of an overall polygenic component to NIDDM.