The binding of ²¹¹At-7G7/B6 to IL-2Rα on Kit-225-IG3 cells was specifically inhibited by unmodified 7G7/B6 mAb. A competitive binding assay was performed as described in “Binding integrity of radiolabeled antibody” under “Materials and methods.” The line with diamonds shows the binding of ²¹¹At-7G7/B6 mAb to IL-2Rα–expressing Kit-225-IG3 cells at 3 × 10⁵, 1 × 10⁶, and 3 × 10⁶ cells. The point with the square symbol is the ²¹¹At-7G7/B6 mAb binding to IL-2Rα when blocked by the addition of a 100-fold greater concentration of unmodified 7G7/B6. The line with triangles represents ²¹¹At-11F11 mAb binding to the IL-2Rα–expressing Kit-225-IG3 cell. cpm indicates counts per minute.