SUMMARYStrain 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 designatedH‐12;this usage is now confirmed. The allele of B10.129(12M) is assigned the symbolH‐12b. Congenic resistant strain B10. 129(6M) is alsoH‐12bbut in addition carries a minorH‐2difference from B10. Strain B10 isH‐2b; strain B10. 129(6M) has a newH‐2allele, derived from the 129 parental strain, to which the symbolH‐2bcis assigned. Strain B10. 129(6M), unlike B10, also reacts with TL antisera. Presumably theTlaallele of 129, which determines the TL.2 antigen, was brought in with theH‐2allele to which it is closely linked.