New data are presented concerning a blood group locus of mice originally identified by Hoecker and co-workers. Specificity “R”, best defined by a (BIO X LP.RIII) anti-RIII, and specificity “Z”, best defined by an RF anti-C3H, are shown to be antithetical. Specificity R is present in strains RF, RIII, and F, absent from all other strains tested; specificity Z is absent from RF, RIII and probably F, and present in all other strains tested. Tests in a segregating generation show that R and Z are determined by alleles at a single locus. The locus is distinct from and probably not linked with H-2, though distorted ratios have been observed in certain cases. Evidence with respect to distinctness from other histocompatibility and blood group loci is incomplete, though there is a considerable presumption that it is not the same as any of the 10 loci identified by strictly histogenetic methods. Strong anti-R sera are easily prepared; specificity Z is somewhat weaker.