The structural organization of the H-2 complex, particularly in the telomeric class I regions D, Q, and TL, is subject to considerable variation from one haplotype to another. In the H-2b and H-2k haplotypes, and presumably in most other haplotypes, the D region consists of a single class I locus ). In the H-2d and H-2q, and presumably in H-2v, the D region contains five class I loci, D, D2, D3, D4, and L (Stephan et al. 1986;). Thus, the H-2D region appears to have two basic types of configuration. However, as additional haplotypes are analyzed in detail, evidence for the existence of other configurations begins to emerge Tummuru et al. 1992). Recently, we located a mouse homologue of the human BAT1 locus, one of the new HLA genes found in the class Ill region of the HLA complex, in close proximity to the Db gene ). The distance between the Tnfgene cluster and the mouse Bat-1 locus is about 40 kilobases (kb), as in the HLA complex, suggesting that this interval is well preserved among species. In contrast, the interval between Bat-1 and the closest distal class I locus is about 10 kb in the H-2b, as opposed to 170 kb in the HLA complex. Thus, it seemed likely that the known difference of approximately 10 kb in the size of the Tnf cluster/D locus interval in the H-2b ) and the H-2ct (Mtiller et al. 1987) reflects a variability in the Bat-1/D interval. The five locus configuration of the H-2D d region may have been derived from an unequal crossover event, as proposed earlier ). According to this hypothesis, the L locus was derived from the D locus. This proposal rests on the strong sequence similarities between the flanking sequences of D a and L d on