A new histocompatibility locus, H-13, in linkage group V is described. The locus is identified by the congenic strain pair C57BL/10ScSn and B10.129(14M). It is moderately “strong” as compared with other non-H-2 loci. The order of the genes in linkage group V used in this study is a H-13 un we H-3. There is some evidence suggesting a possible third, rather weak histocompatibility locus between H-13 and H-3. There is also evidence suggesting interactions between the histocompatibility loci in this region. Whereas transplants from C57BL/10 to 14M (H-13a to H-13b in the presence of H-S‘) are strongly resisted, transplants from B10.LP-a to B10.LP (H-13a to H-13b in the presence of H-3b) are accepted with scarcely a trace of resistance. This has been demonstrated by both skin grafts and marrow transplants.