Quantitative biodistribution was used to assess the in vivo targeting performance of F8-IL2 (n = 4) as described previously (26 (link)). Purified F8-IL2 was radiolabeled with iodine 125 (Perkin Elmer) using the Iodogen method. Immunocompetent 129/Sv mice (n = 3) were injected with 12x 107 F9 teratocarcinoma cells subcutaneously into the right flank. Mice were monitored daily, and tumor volume was measured with a caliper. Tumor volume was calculated as follows: (length [mm] x width [mm] x width [mm])/2. When the tumors reach approx. 350 mm3, 0.86 μg of radio-iodinated F8-IL2 was injected intravenously into the lateral tail veins. After 24 hours, mice were sacrificed, tumor, liver, lung, spleen, heart, kidney, intestine and blood collected, weighed, and the radioactivity was counted using a Cobra γ counter (Packard, Meriden, CT, USA). The radioactivity of tumors and organs was expressed as percentage of injected dose per gram of tissue (% ID/ g ± SD).