INTERFERENCE during the replication of vesicular stomatitis virus (VSV) was first described by Cooper and Bellett1, who suggested that the interfering activity which was present in viruses collected after several successive undiluted passages resulted from a transmissible component. Although they did not identify the component, evidence was presented that it was not the virion. Cantell et al.2 obtained interference whenever a high multiplicity of infection was used, irrespective of the passage history of the virus. Studies in this laboratory3 have shown that viruses collected from both high and low multiplicities of infection possess interfering activity, although repeated undiluted passage of virus is more likely to give a product with marked interference effects. These studies also showed that the interference does not result from interferon but is probably caused by an incomplete form of the virus.