Hemadsorption-Negative Plaque Test: New Assay for Rubella Virus Revealing a Unique Interference tion-negative plaque technique

A simple and rapid plaque procedure has been developed for detecting and accurately assaying rubella virus in a noncytopathic virus-cell relationship. Plaque-formation is based on the development, in individual cells infected with rubella virus, of a unique type of intrinsic interference to infection with Newcastle disease virus. Rubella virus—infected cells challenged with Newcastle disease virus and tested for hemadsorption 15 hours later stand out as hemadsorption-negative areas. Individual living cells infected with rubella virus can be resolved under conditions allowing standard cloning procedures. In principle, the hemadsorption-negative plaque test can be used to search for a new class of noncytopathic, non-hemadsorbing viruses—those that induce an intrinsic interference to infection by any hemadsorbing virus.