Antibody-based therapeutics are powerful tools to treat disease. While their mechanism of action (MOA) always involves binding to a specific target via the Fab region of the antibody, the induction of effector functions through the Fc region of the antibody is equally important for antibody therapeutics designed to deplete tumor cells. By binding of the Fc region to Fc gamma receptors (FcγRs) on the surface of immune cells or complement factors, antibody therapeutics exert effector functions such as antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC), both of which induce target cell death and aid in the efficacy of treatment. Another major Fc effector function is antibody-dependent cellular phagocytosis (ADCP). ADCP is the mechanism by which antibody-opsonized target cells activate the FcγRs on the surface of macrophages to induce phagocytosis, resulting in internalization and degradation of the target cell through phagosome acidification. ADCP has been implicated as a major MOA of several biologics, but this activity is difficult to measure in in vitro. Most assays measure the association of target cells and macrophages; however, co-localization can represent cell attachment rather than internalization. Here, we describe the development of a novel method to accurately measure ADCP activity. By labeling target cells with a pH sensitive dye that only fluoresces in mature phagosomes, the ADCP activity of antibody therapeutics can be accurately quantitated via flow cytometry.