Summary: The ability of a virus to bind to and enter a target cell is a critical component of the viral life cycle. Detailed understanding of virus-cell interactions greatly improves our ability to utilize viruses as tools in both basic research and in clinical settings for gene therapy. The research presented here is aimed at furthering the understanding of virus entry focusing on receptor identification, mechanisms and enhancement of virus fusion, and overcoming a cellular defense to retrovirus infection. Our initial work focused on vesicular stomatitis virus (VSV), which is commonly used in both basic research and as a virus vector for gene therapy. We have demonstrated that a phospholipid, phosphatidylserine (PS), is not the cellular receptor for VSV as previously assumed. We demonstrate that there is no correlation between cell surface PS levels and VSV infection or binding. Furthermore, an excess of annexin-V, which binds specifically and tightly to PS, does not inhibit infection or binding by VSV. During the course of this work, we discovered that treatment of target cells with PS liposomes can increase enveloped virus infection by up to 20-fold. This effect does not bypass virus entry receptor requirements and is not observed with phospholipids other than PS. We provide evidence that this phenomenon occurs through enhancement of virus fusion and we characterize the utility of this technique as a tool to facilitate virus infection. In addition to enhancing normal infection, we have demonstrated that PS treatment can allow certain retrovirus vectors to bypass glycosylation-blocked receptors in some cell types. The implications of this finding for the current model of receptor glycosylation are discussed. In summary, this research provides new insight into the study of virus entry, in particular into the roles of phosphatidylserine in this process.