This project will aim at developing novel methods of immunisation of marine fish, cod and halibut, during early life cycle stages. The approaches will come from a better understanding of the invading virus and the host's response to the infection. Focus o n virulence factors of the virus will result in a better strategy for designing attenuated or characterising avirulent viruses for use in immunisation. Parallel studies of martality following challenge with detailed information about the nucleotide sequen ce of major structural proteins, will aid in identifying virus strains with lost virulence and with corresponding changes at the nucleotide and peptide level. These studies will go hand in hand with conventional methods using inactivated virus. As uptake and distribution to circulation is believed to be a limiting factor as regards efficacy of inactivated vaccines, spedific attention will be paid to identifying methods of inactivation that preserve the viral proteins of importance for uptake into cells on the fish surface, like gills. The ultimate goal is to develop immunisation strategies for early life cycles stages of cod and halibut and thus increase survival rate and economic viability of the industry.