A protein of 34 kDa (Nx) was induced in vitro by the infection of fish cell cultures with the rhabdovirus agent of viral haemorrhagic septicaemia (VHS) of the trout. This protein only appeared as a major component in concentrated or intracellular labeled VHS virus but not in purified VHS or in the related infectious haematopoietic necrosis virus. That Nx protein is antigenically related to the nucleoprotein of purified virus was shown by its reaction with four anti-nucleoprotein monoclonal antibodies (at least 3 of them reacting non competitively against different epitopes) and by immunoprecipitation with polyvalent international reference sera. The Nx protein was shown to be specifically associated with free non-infective particles isolated by ultracentrifugation which were confirmed to be nucleocapsids by electron microscopy.