The infectious hematopoietic necrosis virus (OSV strain) and the virus of haemorrhagic septicaemia of rainbow trout (Salmo gairdneri) (Denmark strain) were examined for possible antigenic relationship by plaque neutralization using homologous and heterologous antisera. No neutralization of either virus was observed on exposure to heterologous antiserum. This indicates that there is no antigenic relationship between these two viruses as determined by this method.