Abstract
A multiplicity-dependent interference was observed in respiratory syncytial virus preparations (Randall strain) grown in HEp-2 cells, and the factor mediating this interference was characterized. Cloned virus did not demonstrate this multiplicity-dependent interference, but its replication was shown to be inhibited by the interfering factor by assays of reduction of infectious yield assay, the interferon factor was found to be particulate, to be inactivated by UV irradiation, and not to interfere with the replication of a heterologous virus, vesicular stomatitis virus. These characteristics are compatible with the physical properties and biological behavior of defective interfering particles. Defective interfering particles were generated by four undiluted passages of cloned virus but were not apparent after eight passages at a multiplicity of infection of 0.1.