I-J subregion was originally identified by a locus, Ia-4, which controls a cell surface molecule on suppressor T cells (Ts). By the occurrence of two pairs of recombinants, B10.A(3R) and B10.A(5R), and B10.S(9R) and B10.HTT, which differ in the haplotype origin of Ia-4, I-J subregion was defined as a segment intercalated between I-B and I-E subregions. It has been shown that the suppressive T cell factor (TsF) is derived from Lyt-2+, 3+ T cells and can be absorbed by anti-I-J antisera. The acceptor site for the TsF on Lyt-1+, 2+, 3+ cells is also blocked by anti-I-J. A question remains to be solved as to whether or not the TsF and acceptor are coded for by the same locus. Moreover, we have recently shown that some but not all helper T cells carry determinants coded for by genes in I-J subregion.