With the help of time-lapse phase contrast photography (1:200 to 1:6000) it is possible to follow HIV-induced cell fusion and the formation of syncytia in monocyte/macrophages isolated from human blood. Syncytia are rarely seen in normal cell culture. The infection of monocyte/macrophages by viruses, for example human immunodeficiency virus (HIV), induces fusion between these cells which results in large multi-nucleated syncytia. In HIV-infected macrophages that have been in culture for a number of weeks these syncytia can reach a significant size and contain dozens of nuclei. The film further demonstrates not only the formation of syncytia through HIV-induced cell fusion, but also specifically the dynamics of this process, the kinetics of cell fusion and syncytia formation as well as the nature of the involvement of macrophage subtypes.