Bacteria-free samples, eg a nasal swab or diseased spleen, combined with antibiotics, are inoculated into animals, eggs or cells, which are known to support the suspected virus. These examined for cell death or lesions in comparison to uninoculated controls and positive controls several days later. Second and third serial passages are often required to increase the level of virus eg to recover a low level of virus from an equine influenza nasal swab in eggs.