The immune system confronts antigens through the variable domains of antigen receptors on lymphocytes. The biochemistry of these antigen receptors is fundamental to the understanding of the immune system. Whilst B cells use immunoglobulins (Ig) as antigen receptors, T cells use unknown molecules which appear to differ from Ig except for the shared variable region of the Ig heavy chain (VH). The notion that T-cell receptors consist, at least in part, of immunoglobulin VH regions stems largely from experiments using anti-idiotypic antibodies1-3. Much serological and genetic evidence has meanwhile accumulated suggesting that VH associated idiotypic determinants are shared between Ig molecules and the antigen receptors of certain classes of T cells including helper cells, cells proliferating on mixed lymphocyte reaction delayed type hypersensitive reactive cells, and cytotoxk T cells 4-7. This pertains to structural elements associated with hypervariable portions of VH, but data on framework elements of VH regions of T cells are scarce8,9. We have previously reported that mouse T-cell receptors react with a rabbit antibody to the V H fragment of MOPC 315 (anti-VH)10-12. The VH framework determinants appeared polymorphic in that the T-helper cells as well as the immunoglobulin heavy (H) chains of a variety of mouse strains reacted equally well, except in the case of strain AKR12,13. We report here the genetic control of the VH framework determinant whose expression through T-helper cells seems to be governed by at least one gene linked to the Igh allotype linkage group. Thus, this VH framework determinant behaves like the rabbit VH region allotypes which are expressed on both immunoglobulin H chains and T-cell receptors 8,9. These data suggest the expression of entire VH regions in T-cell antigen receptors1-3.