Viral pathogens can cause wide-spread pandemics (influenza type A, SARS-CoV-2), may have high mortality rates (Ebola virus) and pose enormous health and socioeconomic risks. Certain viruses can also be used as agents for bioterrorism. Therefore, it is important to develop and manufacture new anti-viral therapeutic modalities. For this purpose, in addition to the antigen ELISA binding assays, commonly used for release and stability testing, it is of particular significance to implement potency assays that allow a more direct assessment of the mechanism of action of the emerging anti-viral therapies. Ideally, these assays should be safe and relatively easy to develop, qualify and validate. Creation of significantly less pathogenic pseudotyped viruses or replication incompetent virus-like particles, expressing only the surface glycoproteins of highly virulent agents, has the potential to serve as a basis for the development of these much needed types of potency assays to assess the activity of new virus entry inhibitors. that assess the ability of the anti-viral therapy to block virus entry into cells. In addition, virus surface glycoprotein induced cell-cell fusion may provide for the development of alternative type of potency assays for evaluation of anti-virus antibodies and antibody related molecules.