The commonly used techniques for viral detection are tedious, time consuming and in many cases inadequate. As a consequence, a rapid, sensitive radioimmunoassay has been developed for detecting viruses. Reovirus is reacted with homologous 125I labeled antibody after which the antigen-antibody complexes are separated from unreacted labeled antibody by density gradient ultracentrifugation. After centrifugation, the density gradient is fractionated and the radioactivity counted in a liquid scintillation spectrometer. The amount of activity in the lower fractions of the density gradient is directly proportional to the virus concentration. The radioimmunoassay developed has several advantages over other viral assay procedures; the method is rapid, viral samples can be assayed within six hours after receiving the sample, both viable and inactive viruses are detected, and the procedure is sensitive.