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Western blot (WB) and eleetroiinmunotransfer blot (EITB) methods were used to monitor baeterial soluble antigens and seroconvcrsion profiles, respectively, in Atlantic salmon, Salmo salar L., experimentally challenged with Renibacterium salmoninarum. The two challenged populations exhibited differences in susceptibility to R. salmoninarum permitting an evaluation of the methods in fish populations harbouring different levels of the pathogen. WB detection of R. salmoninarum soluble antigens in serum provided a sensitive method for detceting pathogen presence during the aetive infection stages. For comparison, results for drop-plate culture and direct fluorescent antibody techniques were obtained. During the recovery phase, surviving fish were negative for the pathogen by these methods. However, >90% of challenged fish sampled from early infection stages to the end of the experimental period (141 days post-infection) were seropositivc by the EITB assay for Atlantie salmon immunoglobulin reaetive with R. salmoninarum soluble antigens. The results suggest that these methods for detecting R. salmoninarum or its soluble antigens may be useful during active infeetions. However, asymptomatie fish populations exposed to the pathogen, perhaps having recovered from an active infection and representing potential disease earrier sources, may be more readily identified by antigen-speeifie antibodies using methods such as the EITB assay.

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