Abstract.Using Western blot to examine the nature of soluble antigens produced byRenibacterium salmoninarum, it was found that the major 57‐kilodalton (kDa) antigen was unstable, SDS‐PAGE of extracellular product (ECP) fractions showed that degradation of the 57‐kDa protein increased with time and increased temperature. Several lower molecular mass pcptides accumulated temporarily from this degradation. Phenylmethylsulphonyl fluoride prevented breakdown of the 57‐kDa protein suggesting a scrine protease present in the ECPs was responsible. The results indicated that most, if not all, immunoreactive bands in ECP fractions, other than the 57‐kDa protein, arose as a result of degradation of this protein. Western blot analysis of two dimensional gels revealed that the presumptive proteolytic activity was associated with the 57‐kDa antigen and several of the apparent degradation products. Many common peptide fragments appeared to be generated from heat‐induced proteolysis of these protein moieties, confirming the familial relationship between much of the immunoreactive material in ECP fractions. The results suggested that the 57‐kDa antigen is autolytic. Western blot analysis of tissue samples from Atlantic salmon,Satmo solarL., infected withR. salmoninarumsuggested that this lability of the 57‐kDa antigen als