Diabetes-prone (DP) BB rats spontaneously develop insulin-dependent diabetes resembling type 1 diabetes mellitus in man. They also exhibit lifelong T cell deficiency. The segregation of both diabetes and lymphopenia was studied in crosses between this inbred line of rats and the related but nondiabetic and nonlymphopenic inbred diabetes-resistant (DR) BB rat line. Diabetes segregated as a single, autosomal recessive trait and was always accompanied by lymphopenia. Among the limited number of differences in the genomic DNA sequences of the two lines, DP and DR BB, one may account for the development of diabetes and lymphopenia in the DP BB rats. It may be possible to screen the genomic DNA for such differences to detect a marker for the phenotypes.