Direct optical trapping of single viral particles allows characterization of individual particles in suspension with singlemolecule sensitivity. Alternative to direct optical trapping of particles, individual particles may be tethered specifically in suspension for manipulation by optical tweezers indirectly, which could be useful for studies of virus-cell interactions. One specific example is the interactions between cell surface receptors and the envelope glycoproteins (Env) on the surface of human immunodeficiency virus type 1 (HIV-1). Env binds to cellular receptors and undergoes a series of conformational changes, culminating in fusion of the viral and cellular membranes that mediates viral entry into cells. In addition to being required for cellular infection, Env is also the sole target for neutralizing antibodies. Thus, significant research has focused on elucidating the structure of Env and the mechanism of HIV-1 entry. However, current methods are unable to resolve the dynamics and stoichiometry of Env binding to cellular receptors during the entry process. Fluorescence and electron microscopy have visualized Env clusters in the viral membrane, but the extent to which these clusters actually bind to cellular receptors, and the mechanism of cluster formation, remain unclear. We describe the development of an optical tweezers technique that can potentially address these questions by delivering a single HIV-1 virion to a live cell with minimal perturbation to the system. Our method can be used to quantitatively probe the physical interactions between Env and cellular receptors in their native environment, which may reveal critical parameters in HIV-1 entry. Furthermore, our method can be used to investigate other protein-protein interactions in the context of live cells, such as the recognition of particulate antigens by B cells, thus offering insight into fundamental features of proteinmediated receptor activation.