Such defective particles may interfere with the ability of non-defective particle to establish a successful infection. Uses indirect fluorescent antibody staining. Infected cells are diluted onto a monolayer of susceptible cells and covered with agarose. Transformation assay used to assay viruses that do not form plaques but can transform. E.g. Rous sarcoma virus in chick embryo fibroblasts.
Most viruses can be easily seem under electron microscopy.
Hemagglutination assay Utilizes the fact that many viruses can bind and link erythrocytes of certain animals. The assay involved determining the number of radioactivity incorporated into the nucleic acids. These technique take advantage of the specificity of antiviral antibodies to the antigen. Some antiviral antibody can bind on the viral particles and neutralize its infectivity. Virus neutralization assay is conducted by mixing dilutions of serum or monoclonal antibody with virus, incubating them, and assaying for the remaining infectivity with cell culture, embryonated eggs, or animals.