The characterization of the antigen-recognizing structure on the various classes of T cells has been a difficult problem for immunologists. Three aspects of this problem are particularly controversial: first, whether the genes that code for the variable portion of immunoglobulin also code for the antigen-recognizing portion of the receptor on all classes of T cells (1-4); second, whether T cell receptors specific for allo-major histocompatibility complex (MHC) 1 products are unique or are included within the population of receptors for conventional antigens (5-10); and third, how T cells, particularly those of the cytotoxic and helper classes, are able to demonstrate an apparent dual specificity for both a conventional antigen and a product of the MHC such that they function only when confronted with both the appropriate antigen and MHC product (11-16). Various models have been proposed to explain this phenomenon. Some have proposed that antigen and MHC are recognized independently via two receptors on the T cell (17-23). Others have proposed various forms of dependent recognition, involving, for example, a single receptor on the T cell, and/or a physical interaction between antigen and MHC products in antigenpresenting cells (24)(25)(26). A number of experimental results have been used by various investigators to argue for one or the other of these models, but there is little evidence that distinguishes the various possibilities.A direct attack on these questions has been hampered by the lack of suitable sources of clonal T cell lines analogous to the B cell myelomas. Recently, two techniques have offered promise in this regard. The first is the establishment of long-term cultures of normal antigen-specific T cells through the use of repeated in vitro antigen stimulation and/or the growth factor, interleukin 2 (IL-2; formerly, T cell growth factor) (27-30). The second is the production of immortal T cell hybrids between normal antigenspecific T cells and T cell tumor lines (31-36). In the present study, we combined these two methods to produce inducible, antigen-specific, H-2-restricted T cell hy-*