Murine splenocytes immune to influenza virus-activated human T-cells were fused with SP2/0 cells, selected in chemically defined HAT media, and subcloned to yield a monoclonal antibody (MAb) termed 7G7/B6. 7G7/B6 binds to lectin- and antigen-activated T-cells, but not resting T-cells or B-lymphoblastoid lines from the same donor. 7G7/B6 immunoprecipitates a 50-55 kD band from cell surface iodinated PHA-activated T-cells or the T-cell leukemia line HUT 102B2, as shown on SDS-PAGE. Cross-clearing studies demonstrate that 7G7/B6 binds the same cell surface molecule(s) as anti-Tac, a MAb which has been shown previously to recognize the human receptor for IL-2. 35S-methionine pulse chase experiments in HUT 102B2 cells reveal that 7G7/B6 binds to an early (less than 30 min) 35-37 kD and late (greater than 4 h) 50 kD protein. Sequential immunoprecipitations demonstrate that these are identical to the molecules identified by anti-Tac under similar conditions. However, only anti-Tac coprecipitates a higher molecular band at 110 kD. 7G7/B6 and anti-Tac do not competitively inhibit the binding of each other to PHA-activated T-cells. Functional studies reveal that in contrast to anti-Tac, 7G7/B6 has almost no inhibitory effect in vitro on IL-2-driven proliferation of IL-2-dependent T-cell lines, or alloimmune cytotoxic T-cell generation (however, once generated, these cytotoxic T-cells were both 7G7/B6 and anti-Tac positive). Finally, IL-2 does not inhibit the binding of 7G7/B6 to activated T-cells under conditions which result in up to 75% inhibition of anti-Tac binding. Therefore, 7G7/B6 is another MAb recognizing the human IL-2 receptor, but binding to an epitope distinct from that recognized by either IL-2 or anti-Tac.