Infectious haematopoietic necrosis (IHN), a rhabdoviral infection of salmonid fish, was considered to be an exotic disease in Europe until it was recognized in France and Italy in 1987. In France, the existence of this new condition led the authorities in charge of animal health to order epidemiological studies to be undertaken. These studies were based upon virological, serological and experimental diagnostic methods and also encompassed disease eradication attempts. Studies were conducted at 7 fish farming sites, involved 1545 salmonid fish, of which 848 were sacrificed, and represented 262 virological examinations and 1782 serum neutralization tests. The presence of the IHN virus was detected in the 7 trout farm fish populations that were located in 5 regions, one of which was situated 600 km from the place where the first isolation of IHN virus was made. Moreover, 6 out of 7 rainbow trout populations reared in these farms also harboured viral haemorrhagic septicaemia virus (VHSV) often resulting in overt disease. Rainbow trout was the only salmonid fish species found infected with IHN. Over infection, which was observed in fish ageing less than 2200 degrees-days, always occurred at water temperatures below 14 degrees C, and the younger fish were more susceptible (mortality rate greater than or equal to 80%). Although the IHN virus is easily isolable from fish undergoing overt infection, if was hardy detectable in survivors until they were adults, at which stage the virus was shedded via sexual products which constituted suitable materials for virological examination and disease transmission assays. Survivors of overt and dormant IHN infection developed consistent immune response and special attention was paid to neutralizing antibodies (NAb) to IHN virus. The detection of such NAb in fish from infected farming sites or other NAb from presumably IHN-free sites, correlated fairly well with the presence and further detection of IHN virus among such fish populations. Our data provide arguments for considering the serological technique as a suitable means of completing fish health surveillance programmes for IHN. Although our results are in agreement with part of the existing knowledge on IHN, they differed in several points: rainbow trout was the only susceptible fish species; overt IHN was always recorded in juveniles and at water temperatures below 14 degrees C; IHN virus could not be recovered from the mucus of infected broodfish; IHN infection usually coexisted with VHS infection in same fish population, and serology was widely and successfully used for the diagnosis of IHN.