A method for counting Infectious pancreatic necrosis virus (IPNV) through epifluorescence microscopy was analyzed in detail. Image processing and statistic considerations are included. The particle size of viruses was compared in different experimental conditions such as the staining of the virus with SYBR-Green I or with antibodies for specific fluorescence labeling of viral proteins. The type of surface used as mounting support was assayed as well. The results indicated that the most suitable method involves the mounting of the viral-containing suspension on a membrane filter followed by the staining with a monoclonal antibody specific for a viral protein combined with a FITC (fluorescein isothiocyanate)-conjugated secondary antibody.