IL-2R-targeted therapy prevents graft rejection in various experimental models and in man. However, the principles of optimal mAb selection remain elusive, as their efficacy in vivo does not always correlate with their characteristics in vitro. ART-18 and OX-39, mouse IgG1 mAbs, define distinct epitopes on the p55 subunit of the rat IL-2R. Treatment of LEW hosts with ART-18 prolongs survival of LBN cardiac allografts up to a month; in contrast, OX-39 never affects acute (8-day) rejection. In this study, we evaluated the biodistribution of 125I-labeled ART-18 and OX-39 administered iv to untreated and heart-grafted rats. ART-18 was cleared from the circulation (half-life time ca. 29 hr) and accumulated in host lymph nodes and spleen to a greater extent than OX-39 (P less than 0.001). In contrast, OX-39 was retained in blood (half-life time ca. 66 hr) and was eventually sequestered in liver, lungs, and kidneys, a pattern comparable to an irrelevant IgG1 (MOPC-21). ART-18 but not OX-39 entered specifically acutely rejecting allografts (allograft:native heart activity ratio = 4.0 and 2.3, respectively, P less than 0.01). The distribution of ART-18 was IL-2R epitope but not mAb isotype specific as tissue accumulation of "hot" ART-18 was comparable in recipients conditioned with "cold" ART-18 of IgG1 or IgG2b isotype, but not in those treated with OX-39. Thus: (1) the biodistribution of anti-IL-2R mAbs is not random; the mAb "effective" in combating rejection (ART-18) penetrates preferentially host lymphoid tissues and the graft itself, whereas the biologically "ineffectual" mAb (OX-39) is retained in the circulation for prolonged periods; (ii) the epitope of IL-2R defined by the mAb is critical; a mAb may be "captured" by unrelated cells expressing a common epitope in vivo before reaching the related targets, and/or some epitopes may be more accessible than others for iv administered mAbs.