The premise of this work is that there are characteristic constant region gene products on T cell receptors and that these isotypes are important in cellular collaboration mechanisms. It has been shown that VH gene products are shared between T and B cells and that anti-VH antibodies are capable of triggering T cells to enter the regulatory network controlling the synthesis of antibody. The hypothesis outlined here is that VH genes are translocated onto characteristic Igh-Tc genes located between Ig-1 and prealbumin on chromosome 12 and that the function of the recombined gene product is the key to the interaction patterns of T lymphocytes which modulate the cellular wing of the immune response. A serological approach will be used for immuno-precipitation studies of the molecular properties of T cell receptors and for induction of the biological activities attributed to subsets of T cells. The existence of discrete isotypes on T cells and their possible role in triggering and regulation of cell function will be explored and compared with the known isotypes characteristic of the B cell repetoire.