Pretreatment of mouse lymphoid cells with anti-Ia sera and C abrogated the proliferative responses of these cells to Con A. Studies were carried out with several anti-Ia reagents and intra-H-2 recombinant mouse strains to map the I subregion(s) whose products are expressed on Con A-reactive cells. Treatment with a (B10.A × A)F1 anti-B10 reagent and C abrogated the ability of BALB/c cells to respond to Con A. Absorption studies on this reagent demonstrated that Con A-reactive cells express Ia determinants coded by the I-A subregion. The results with two additional reagents, B10.A(4R) anti-B10.A(2R) tested on B10.BR cells and (B10 × D2.GD)F1 anti-B10.D2 absorbed with B10.A cells and tested on BALB/c cells, demonstrated that Con A-reactive cells also express Ia determinants encoded to the right of I-A. Several antisera and strain combinations were evaluated in which the antisera could contain antibodies specific for products of genes encoded by the I-J subregion, but the results were inconclusive. These data demonstrate that there are at least two different I subregions, one in I-A and one to the right of I-A, that code for antigens expressed on Con A-reactive cells.