Challenge of Atlantic salmon Salmo salar by intraperitoneal injection with Renibacterium salmoninarum at low temperature (7-8°C) caused the fish to become asymptomatically infected. Although no overt clinical signs appeared and no deaths occurred for 14 weeks postinfection, most live sampled fish (65% from tank 1 and 55% from tank 2) from weeks 2 to 17 postinfection had low levels of R. salmoninarum, as indicated by colony-forming units (cfu) per gram of kidney tissue cultured. Only 5-6% of kidney samples from tank-1 fish and none from tank-2 fish tested positive (and those were only weakly positive) for R. salmoninarum soluble antigens by Western blot. Even though the fish holding temperature was increased to 10-11°C on week 13 postinfection, only a low number of fish died (14 in tank 1 and 5 in tank 2) from weeks 15 to 29. By weeks 27 and 31 postinfection, no live sampled fish had detectable R. salmoninarum cfu per gram of kidney tissue cultured. After 31 weeks postinfection, only 4% of live sampled fish from the two tanks produced antibody responses to the R. salmoninarum soluble antigens at levels detectable by electroimmunotransfer blot (EITB). Analysis of commercially reared Atlantic salmon from the Bay of Fundy demonstrated that 32% of the fish were asymptomatically infected, but Western blot and EITB results for these fish were similar to those for the low-temperature experimental challenge fish. These results are discussed in terms of the lack of antibody response to R. salmoninarum and detection of infection in asymptomatic Atlantic salmon.