Juvenile rainbow trout Oncorhynchus mykiss were experimentally infected by immersion with viral haemorrhagic septicaemia virus (VHSV), infectious haematopoietic necrosis virus (IHNV) or with both viruses. The presence of neutralizing antibodies in the sera of infected fish were analysed by 50% plaque neutralization tests (50%PNT). In Group 1 (infected with VHSV) and Group 2 (infected with IHNV) neutralizing antibodies were found in 41% and 21% of the serum samples, respectively. No cross-reacting antibodies were found in these 2 groups. In Group 3 (infected with both viruses) 30% of the samples showed neutralizing antibodies against VHSV, 21% against IHNV and 12% against both viruses. Fish in Group 3 developed a double specific antibody reaction whose kinetics and intensity (mean of log10 titres) were similar to the antibody response of the single infected groups.