The tissue distribution of 89Zr-MEHD7945A was assessed in mice-bearing BxPC-3 tumors. A tracer dose of 10–15 μCi (370–555 kBq, 1–2 µg, 6.7–13.3 pmol) was injected i.v. on the lateral tail vein. In vivo competitive specificity assays were conducted in separate cohorts (n = 4–5 per group) of BxPC-3 and AsPC-1 xenografts. The tracer dose 25–30 μCi (0.925–1.11 MBq, 5–6 µg, 33.3–40 pmol) was co-injected with 10-fold higher blocking doses of either MEHD7945A (50 μg), cetuximab (50 μg) or the anti-HER3 mAb, DL3.6B (50 ug). Euthanasia via CO2 asphyxiation was performed between 24–120 h post injection (p.i.) of the tracer. For the in vivo competitive binding assay, mice were sacrificed 48 h p.i. Blood was immediately collected via cardiac puncture. Select organs were harvested, rinsed and dried to remove excess water. Bound activity was measured using a gamma counter. Activity measurements were background- and decay-corrected to the time of counting. The tissue uptake, expressed as % injected dose per gram of tissue was calculated. Western blots and immunohistochemistry (IHC) validated the tumor uptake of the tracer.