Antisera (B10.129×A)F1 anti-P and (B10×A)F1 anti-B10.P contain antibodies that define, in the PVP hemagglutination test, an antigen originally described as G or H-2.7. Of the independent H-2 haplotypes, the H-2.7 antigen is present in f, j, k, p, and s. In addition, the antisera also contain a weak cytotoxic antibody, distinct from anti-H-2.7. The cytotoxic antibody reacts with antigens controlled by the K or I regions. The hemagglutinating H-2.7 antibody does not have cytotoxic activity. The genetic determinant coding for antigen H-2.7 can be mapped into the chromosomal segment between the S and D regions. The H-2.7 antigen thus serves as a marker for a new region of the H-2 complex. The locus coding for antigen H-2.7 is designated H-2 G and the corresponding H-2 region G. The H-2.7 antigen has a tissue distribution distinct from that of the H-2 antigens controlled by the K or D regions. So far it could be detected primarily on erythrocytes. © 1975 Springer-Verlag New York Inc.