Abstract. Hetereogeneity of rainbow trout immunoglobulins was demonstrated by using monoclonal antibody 1A6 and polyacrylamide‐gradient gel electrophoresis. Immunoglobulins defined by elisa using monoclonal antibody 1A6 were about 30% of the total immunoglobulins, detected by elisa using polyclonal antibodies, in healthy rainbow trout. In trout obtained from farms with a previous history of infectious viral diseases, 1A6‐immunoglobulins were only about 14% of the total. Several serum pools from infected trout could be totally depleted of 1A6‐immunoglobulins (about 12% of total immunoglobulins) by affinity chromatography over Sepharose immobilized monoclonal antibody 1A6. Polyacrylamide‐gradient gel electrophoresis under denaturing conditions of total immunoglobulins, 1A6 immunoglobulins and no‐1A6 immunoglobulins purified by affinity chromatography, showed a majority heavy chain of 70 KDa and a minority heavy chain of about 60 KDa, two light chains of 24 and 26 KDa, and a 11–14 KDa polypeptide.