The partially inbred strain of BB-Wistar rats showed a varying incidence of the insulin-dependent diabetes-like syndrome. The serological typing of a large sample of BB rats verified the homozygosity for the RTlu haplotype, whereas its parental non-inbred Wistar stock segregated for RTlu and RTla haplotypes. The histogenetical typing of BB rats by skin grafting showed a significantly prolonged rejection of grafts from RTla donors unusual in other RTlu recipients. The presumption of some recombinational or mutational events in the RTl haplotype of BB rats was not verified by the simple F1 skin grafting test from LEW.1U/RTlu standard/donors to F1/LEW X BB/recipients. Skin grafts survived permanently. When trying to get a clear-cut answer whether the RTlu haplotype is associated with the spontaneous occurrence of diabetes in F2/LEW X BB/ X /LEW X BB/hybrids, only 7.4% of RTlu homozygotes were found among 359 weaned animals. Moreover, the partial strain of BB rats became extinct with the F1 generation mainly due to an infection by the mycoplasma. A new sample of outbred BB rats with a low incidence of spontaneous diabetes was found as homogeneous for the RTlu haplotype, too. Preliminary results of typing these rats for the secondary antibody response to pig insulin indicate the low responsiveness contrary to the results given earlier for the RTlu typing strains as high responders. All these results support the idea that the RTlu haplotype of the BB rats might be a variant carrying some mutational change(s) at the RTl.B region.