The alloantiserum was raised in BALB/c (H-2d, Igh-1a) mice hyperimmunized with spleen cells of Igh allotype congenic mice, CB-20 (H-2d, Igh-1b). It was found to define the new allotypic determinants (distinct from B cell Igh constant region determinants: Igh allotype) expressed only on a small population of T cells belonging to the Thy-1 dull-stained Lyt-2- or Lyt-2+ population of Igh-1b mice. Genes coding for the determinants were shown to be accommodated somewhere in the right side of the Igh variable region gene (Igh-V) cluster, as the antibody activity was completely absorbed with BAB-14 thymocytes. It was also demonstrated that the products detected by the antiserum represent the allotypic determinants (probably constant region determinants) on the antigen-binding moiety of the antigen-specific augmenting (TaF) and suppressor (TsF) T cell factors. Moreover, determinants on TsF were found to be distinct from those on TaF. Therefore, it can be suggested that the two genes coding for the T cell allodeterminants (distinct from those of the B cell Igh) are located in the right side of the B cell Igh-V on the 12th chromosome, and that both encode the antigen-recognition units of the functionally distinct T cell factors.