The locus of the gene that codes for the antigen-specific suppressive T-cell factor was determined to be in a new subregion "I-J" which locates between I-B and I-C subregions in the H-2 histocompatibility complex. This was shown by two different lines of evidence: (a) The absorbing capacity for the suppressive T-cell factor of several alloantisera against restricted I subregions did not correlate with their specificity for previously known Ia molecules which are coded for by genes in I-A and I-C subregions, but was associated with the specificity for the products of genes putatively present between I-B and I-C subregions. By the occurrence of special recombinant strains, i.e. B10.A(5R), B10.A(3R), B10.S(9R), and B10.HTT, which differ with respect to the I-J subregion, we were able to produce alloantisera which distinguish I-J subregion gene products. The absorption studies using these special alloantisera directed to I-J subregion clearly indicated that the suppressive T-cell factor is a product of I-J subregion gene(s), and that the molecule is distinct from known Ia molecules expressed on splenic B cells. (b) Taking advantage of the fact that there is a strict histocompatibility requirement for the effective suppression between the donor and recipient strains of the suppressive T-cell factor, we were able to determine the required identities of the genes in the H-2 complex existing among those present between I-B and I-C. Again, utilizing the T-cell factors obtained from special recombinant strains, i.e. B10.A(4R) and B10.A(5R), we were able to locate the gene that codes for the suppressive T-cell factor reactive only with relevant haplotype strains between I-B and I-C subregions. These results are most reasonably explained by the presence of a new subregion I-J which is specialized in coding for the suppressive T-cell factor as a different molecule from previously known Ia molecules.