A Sensitive Method for Quantification of Vesicular Stomatitis Virus Defective Interfering Particles: Focus Forming Assay

A focus-forming assay for the quantification of defective interfering (DI) particles of vesicular stomatitis virus (VSV) is described. This assay is based on the procedure described for lymphocytic choriomeningitis (LCM) virus by Popescu et al. (1976). Under appropriate conditions this focus-forming assay can quantify fewer than 100 DI particles/ml in a preparation containing a large number of infectious particles.