A rapid, high-throughput, and quantitative method for cell entry route characterization is still lacking in nanomedicine research. Here, we report the application of imaging flow cytometry for quantitatively analyzing cell entry routes of actively targeted nanomedicines. We first engineered ICAM1 antibody-directed fusogenic nanoliposomes (ICAM1-FusoNLPs) and ICAM1 antibody-directed endocytic nanolipogels (ICAM1-EndoNLGs) featuring highly similar surface properties but different cell entry routes: receptor-mediated membrane fusion and receptor-mediated endocytosis, respectively. By using imaging flow cytometry, we characterized their intracellular delivery into human breast cancer MDA-MB-231 cells. We found that ICAM1-FusoNLPs mediated a 2.8-fold increased cell uptake of fluorescent payload, FITC-dextran, with a 2.4-fold increased intracellular distribution area in comparison with ICAM1-EndoNLGs. We also investigated the effects of incubation time and endocytic inhibitors on the cell entry routes of ICAM1-FusoNLP and ICAM1-EndoNLG.Our results indicate that receptor-mediated membrane fusion is a faster and more efficient cell entry route than receptor-mediated endocytosis, bringing with it a significant therapeutic benefit in a proof-of-principle nanomedicine-mediated siRNA transfection experiment. Our studies suggest that cell entry route may be an important design parameter to be considered in the development of next-generation nanomedicines.NANOMEDICINES have emerged as a revolutionary therapeutic approach for treating many diseases including cancer (1-3). They take advantage of rapidly developed nanomaterials to engineer "virus-like" nanovectors that circulate in the body and deliver a variety of therapeutic, diagnostic, and theranostic agents to disease sites. To date, more than 20 nanomedicines (e.g., Doxil, Abraxane, and Onivyde) have been approved by the United States Food and Drug Administration (U.S. FDA) and the European Medical Agency (EMA) for clinical indications due to their fewer adverse effects and better safety profiles than conventional drug formulations(4,5). Moreover, actively targeted nanomedicines (e.g., MM310 and BIND-014) have been under intense pre-clinical and clinical investigations as next-generation targeted therapeutics for cancer treatment. These nanomedicines utilize molecularly targeting ligands (e.g., antibodies, peptides, and aptamers) to guide nanovectors to precisely recognize and ablate cancer cells while sparing normal organs and tissues (4).Cell entry route plays a critical role in the intracellular delivery of actively targeted nanomedicines. In general, nanomedicines are hypothesized to enter cells through two major routes: endocytosis and membrane fusion (6,7). Most actively targeted nanoparticles rely on receptor-mediated endocytosis to enter targeted