Investigation of the biological activity of viruses in vitro necessitates some means of identifying their presence within cells and assaying their activity. Since virus particles themselves are metabolically inert, their detection and quantitation is dependent on their cellular effects or on synthesis of viral antigens, enzymes, and the like. Virus-induced cell fusion is a cytopathic cellular response easily recognizable by microscopic examination, which has been exploited to considerable advantage in virology, as will be briefly discussed.