Challenge experiment to examine virus infectivity in vivo in Atlantic salmon smolts. Hearts from 30 of these smolts were examined by real-time RT-PCR prior to the experiment to ensure that they were not infected by ISA virus. Water samples from 5 fish and all time points (T0, T1, T3, T6, T24, T48 and T96/T120) were injected into five fish for each time point. Fish were selected on the basis of results from the decomposition part of this study. A volume of 0.2 mL of the eluted water sample was injected into each fish. Naïve cohabitating fish were added to the tanks as control fish and these were given EMEM only. The purpose of adding these fish was to control the possible horizontal transmission between fish in the tanks. Thus, each tank contained 20 fish plus 10 control fish (n total per tank = 30). The exceptions were two tanks wherein only 15 fish were i.p. infected with water samples (n total per tank = 25). Tissues (hearts and kidney) were sampled at 7 (2 fish from each group) and 14 (3 fish from each group) d.p.i., respectively.