Purpose: To quantify infectious viruses and evaluate the infectivity of a viral sample.Summary: 50 µL of serially-diluted (10-3 to 10-10) virus sample is added to 150 µL of exponentially growing host cells in triplicate 96-well microplates and incubated at normal growth conditions for ~2 weeks. Cell lysis is assessed every few days qualitatively by visual inspection and quantitatively by measuring optical density on a microplate reader. The MPN of infective viruses in each concentrate is estimated from the proportion of virus-positive (i.e., lysed) wells using the MPN_ver4.xls Excel spreadsheet from Jarvis, B., Wilrich, C., and P.-T. Wilrich (2010), Journal of Applied Microbiology 109 (2010), 1660 – 1667. Percent infectivity is then calculated by comparing the MPN-estimated abundance of infective viruses to the abundance of virus-like particles (VLPs) determined by flow cytometry analysis.