Vesicular stomatitis virus (VSV) is the prototype rhabdovirus. It is an important animal pathogen that causes a disease, which is difficult to differentiate clinically from that caused by foot-and-mouth disease virus. Its broad host range, rapid lytic growth cycle, unique morphology and susceptibility to interferon have all contributed to its popularity as a model system in experimental virology. VSV has been employed frequently for the rapid assay of other more fastidious viruses or for detection of noncytopathogenic viruses exploiting the phenomenon of pseudotype formation. Coinfection of the rapidly cytolytic VSV and a noncytolytic enveloped virus, such as human immunodeficiency virus (HIV), generates an appreciable frequency of pseudotype particles in which infectious VSV nucleocapsids are enveloped by the envelope proteins of HIV. The yield of cytolytic virus after exposure to high-titre anti-VSV serum can detect and quantitate the presence of noncytolytic (e.g. HIV) virus.