We describe here the properties of mAb GK1.5, which recognizes a cell surface molecule designated L3T4; the determinant on L3T4 recognized by mAb GK1.5 is designated L3T4a. We present evidence here that: i) the expression of L3T4a by murine T cell clones correlates primarily with class II MHC antigen-reactivity; ii) mAb GK1.5 blocks all class II MHC antigen-specific functions (cytolysis, proliferation, release of lymphokines) by murine class II MHC antigen-reactive T cell clones, although there appears to be clonal heterogeneity in the degree to which these functions are blocked by mAb GK1.5; iii) mAb GK1.5 blocks class II MHC antigen-specific release of IL-2 from cloned T cell hybridomas by blocking class II MHC antigen-specific binding; and iv) L3T4 is very similar to the human Leu3/T4 antigen. The properties of mAb GK1.5 (complement fixation, reactivity with all mouse strains tested, profound blocking of all class II MHC antigen-specific functions by murine T cells, usefulness for FACS analyses, and usefulness for immuno-precipitation/SDS-PAGE analyses) make it suitable for investigating both the role of class II MHC antigen-reactive T cells in various immunological phenomena and the mechanistic basis, at the molecular level, of class II MHC antigen-reactivity by murine T cells.

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-*

An antibody-secreting B cell hybridoma, KJ1-26.1, has been prepared from mice immunized with the T cell hybridoma DO-11.10, which recognizes chicken ovalbumin in association with I-Ad (cOVA/I-Ad). KJ1-26.1 blocks I-restricted antigen recognition by DO-11.10 and a subclone of this T cell hybridoma, DO-11.10.24, which has the same specificity for cOVA/I-Ad as its parent. KJ1-26.1 does not block I-restricted antigen recognition by any other T cell hybridoma tested, including a number of T cell hybridomas closely related to DO-11.10, with similar, but not identical, specificities for antigen/I. Moreover, KJ1-26.1 binds to DO-11.10 and DO-11.10.24, but not to any other T cell hybridomas tested, including three subclones of DO-11.10 that have lost the ability to recognize cOVA/I-Ad. Thus, in every regard KJ1-26.1 appears to be binding to all or part of the receptors for antigen/I on the T cell hybridoma DO-11.10. KJ1-26.1 appears to bind to approximately 15,000 molecules/cell on the surface of DO-11.10. The antibody precipitates an 80,000 dimer from the cells, which on reduction migrates as 40-44,000 monomers. The receptor(s) for antigen/I on DO-11.10 therefore includes molecules with these properties.