Diabetes-prone (DP) and diabetes-resistant (DR) sublines of the BB rat have been established in Edinburgh, U.K., separately from other existing colonies. In an examination of the lymphoid status of the two lines, BB-DP/Ed and BB-DR/Ed, it has been found that both lines have very low T-cell numbers, depressed B-lymphocyte numbers and a complete absence of peripheral CD8+ T cells, all features characteristic of the previously described genetic lymphopenia lesion. It was also noted that the peripheral T cells of both BB/Ed lines were larger than normal. The DP/Ed and DR/Ed lines were indistinguishable in all these respects, and furthermore, they were both shown to type as RT1u at the major histocompatibility complex (MHC). The genetic combination of lymphopenia and RT1u without expression of diabetes is not present in other extant BB lines and makes BB-DR/Ed a uniquely useful control strain for BB rat studies as well as a valuable genetic resource for the further genetic analysis of diabetes susceptibility in rats.