Internalization of 89Zr-MEHD7945A was evaluated on AsPC-1, BxPC-3 and MIA PACA2 pancreatic cancer cell lines. Wells were seeded with ~1 × 105 cells and incubated overnight. Radiolabeled protein [1 μCi/mL (37 kBq/mL, 20 μg)] in 1 mL of media was added to each well. The plates were incubated at either 37 °C or 4 °C for 0.5–24 h. Following each incubation period, the media was collected and the cells were rinsed with 1 mL 1× phosphate buffered saline (PBS) twice. Surface-bound activity was removed by washing the cells in 1 mL 100 mM acetic acid + 100 mM glycine (1:1, pH 3.5) at 4 °C. The cells were then lysed with 1 mL 1 M NaOH. All washes (media plus PBS, acid and alkaline) were collected in separate tubes and measured for counts using a gamma counter (Perkin Elmer). The %-internalized activity was calculated as the ratio of the activity of the lysate and the total activity collected from the media plus PBS wash, acid, and base washes.