Abstract
The titre of defective interfering (DI) influenza virus measured by an assay based on the inhibition of cytopathology caused by A/WSN (H1N1) influenza virus in MDCK cells was 320,000-fold greater than titres measured by inhibition of infectious centre formation. Interference was less in other types of cell. By electron microscopy, we have shown that the ratio between physical particles and DI units in preparations of the DI virus was approximately unity, which suggested that one or few DI particles is/are required to confer resistance of a MDCK cell to viral cytopathology. This human DI virus interfered heterotypically with an avian H7N1 influenza virus.