A mouse anti-rat interleukin 2 (IL 2) receptor (IL 2R) monoclonal antibody (mAb), ART-65, has been developed. As shown by fluorescence-activated cell sorter analysis and immunoprecipitation studies, ART-65 recognizes in a species-specific manner the same molecule as does ART-18, a mAb which has been shown previously to recognize the rat receptor for IL 2. ART-65 and ART-18 do not competitively inhibit the binding of each other to activated T cells. ART-65, in contrast to ART-18, does not inhibit the binding of IL 2 to cells nor does it have any inhibitory effect in vitro on IL 2-driven proliferation of rat T lymphoblasts. Therefore, ART-65 is another mAb recognizing the rat IL 2 receptor, but binding to an epitope distinct from that recognized by either IL 2 or ART-18. We compared the in vivo activity of the mAb ART-65 and ART-18 with that of the W3/25 mAb in a local graft-vs-host reaction (GVHR). Similar to the anti-W3/25 treatment, ART-65 and ART-18 inhibited GVHR. The results demonstrate that GVHR depends on a small subpopulation of IL 2R+ cells present in the W3/25+ T cell population because IL 2R-targeted therapy was as effective as the treatment with W3/25 mAb which reacts with the entire T helper cell population. Moreover, the results argue against the possibility that anti-IL 2R mAb act via blockade of the IL 2 binding to IL 2R+ cells and/or by inhibiting the IL 2-driven expansion of the antigen-activated clones. The results support the view that IL 2R-targeted therapy results in the elimination of the IL 2R+ cells.