SUMMARY Strain B10.129(12M) is a congenic resistant strain differing from its partner strain, C57BL/10 or B10, by a rather weak histocompatibility locus. The locus is unusual in that the sex difference in the response to grafts is especially pronounced. Preimmunized females are strongly resistant to skin or tumor transplants; preimmunized males are not. Evidence presented herewith shows that B10.129(12M) identifies a new histocompatibility locus. This already has been provisionally designated H-12; this usage is now confirmed. The allele of B10.129(12M) is assigned the symbol H-12b. Congenic resistant strain B10. 129(6M) is also H-12b but in addition carries a minor H-2 difference from B10. Strain B10 is H-2b; strain B10. 129(6M) has a new H-2 allele, derived from the 129 parental strain, to which the symbol H-2bc is assigned. Strain B10. 129(6M), unlike B10, also reacts with TL antisera. Presumably the Tla allele of 129, which determines the TL.2 antigen, was brought in with the H-2 allele to which it is closely linked.