Animal studies using radiolabeled anti-Tac disulfide-stabilized Fv (dsFv) monoclonal antibody have shown formation of complexes in serum with the soluble α subunit of the interleukin 2 receptor α (sIL-2Rα). In this study, we improved the targeting of 125I-labeled anti-Tac dsFv to receptor-positive tumors in the presence of circulating receptor by prein-jecting unlabeled humanized anti-Tac IgG antibody (HuTac IgG). We used mice bearing SP2/Tac tumor xenografts that express the IL-2Rα. A positive correlation was seen between tumor size and the concentration of circulating receptor. Tumor-bearing mice were injected with 125I-labeled anti-Tac dsFv (400 ng), either alone or 15 min after injection of HuTac IgG. The 125I-labeled anti-Tac dsFv formed high molecular weight complexes with the sIL-2Rα. The fraction of the dsFv present in the complexes increased as tumor size increased (greater sIL-2Rα levels). The fractions of dsFv in the complexes were 9.9- to 11.6-fold higher when sIL-2Rα was not blocked with preinjected HuTac IgG. The administration of a 12-fold molar excess of HuTac IgG over sIL-2Rα resulted in >80% of the 125I activity present as the dsFv rather than in the complexes. Furthermore, the biodistribution of 125I-labeled anti-Tac dsFv was improved by blocking its binding to sIL-2Rα by preinjecting HuTac IgG. Specifically, in the preinjected group, at 15 min postinjection, the 125I-labeled anti-Tac dsFv levels in tumor increased to 10.8% compared to 5.6% injected dose per gram in the non-preinjected group. In summary, our studies showed that preinjection of HuTac IgG can block the formation of complexes of circulating sIL-2Rα and 125I-labeled anti-Tac dsFv. This blockade is associated with faster blood clearance, higher tumor uptake, and greater tumor:nontumor ratios of the radiolabeled antibody fragment.