Data presented here show that locidentify in the I-region of the H-2 gene complex are selectively expressed in different functional T-cell subpopulations. These loci are closely linked (or possibly identical) to loci that control immune responses. They control surface determinants which identify helper and suppressor T lymphocytes. Determinants described here on allotype suppressor T cells (Ts) are found on normal (nonsuppressed) lymphoid cells, but are not found on helper T cells (Th). These determinants are controlled by a locus mapping in the I region of the H-2 complex. In an accompanying publication we show that this locus (Ia-4) marks a new I subregion (I-J) and is expressed only on T cells. Thus Ia-4 determinants idenfity a T-cell subpopulation which includes Ts but not Th. Th also carry identifying surface determinants controlled by loci that map to the H-2 complex, probably within the I region. These determinants are not found on Ts. Data presented also establish that loci in the I region control determinants on Th, but do not conclusively demonstrate that these are the determinants that distinguish Th from Ts. The selective expression of H-2-controlled determinants on Ts and Th suggests that these determinants are directly involved in immunoregulation.