Two antibody-secreting murine hybridomas, F1G3.1 and F2A11.5, have been established from B10.D2 mice immunized with cells from the murine cytotoxic T-lymphocyte clone G4. The two clones used, G4 and B10, were derived from BALB.B (H-2b) mice and the target antigen specificity of both maps to the Dd region of the murine H-2 complex. However, B10 has a lower affinity for the target cells, as shown by its lower specific killing of blasts and its higher susceptibility to blocking by anti-Lyt-2 monoclonal antibody 53-6.75. The monoclonal antibodies, F1G3.1 and F2A11.5, react only with cells from clone G4. Similarly, they block only the specific cytolysis mediated by G4; no effect on cytotoxicity mediated by B10 or by heterogeneous populations of cytotoxic T lymphocytes was found. F1G3.1, especially, is very active in stimulating G4 to secrete immune interferon; B10 in contrast did not show any induction on treatment with these monoclonal antibodies. The structure of the surface antigen on G4 cells recognized by these monoclonal antibodies was revealed by immunoprecipitation studies of radioiodinated cell surface proteins. A protein dimer could be identified with an apparent molecular size of 80,000 daltons consisting of monomers migrating as 42,000-dalton proteins on reduction. So far, electrophoresis in the presence of NaDodSO4 does not indicate any heterogeneity in the size of the monomers. This molecule can be distinguished from the Lyt-2 complex.