The expression stability of five commonly used housekeeping genes [beta-actin (ACTB), elongation factor 1-alpha (EF1A), ubiquitin (UBQ), glyceraldehyd-3-phosphate dehydrogenase (GAPDH) and tubulin alpha (TUBA)] were monitored in salmonid cell lines CHSE-214 and RTS11 after infection with two of the most fastidious fish pathogens, the facultative bacterium Piscirickettsia salmonis and the aquabirnavirus IPNV (Infectious Pancreatic Necrosis Virus). This evidence concerns the gene GAPDH and infection.