The humoral antibody response of healthy Atlantic salmon and of two groups of salmon, naturally infected with Aeromonas salmonicida ssp. achromogenes, was examined in some detail. One diseased group was chronically infected and the other recently infected. It was found that the humoral response of these two infected groups was quite different. The chronically infected fish showed poor specific response to the causative agent whereas the recently infected salmon produced strong specific antibody response. The chronically infected fish showed evidence of increased unspecific response including an elevated level of natural antibodies. The specific humoral response of the recently infected fish was primarily directed against two cell-associated antigens of the A. salmonicida ssp. achromogenes bacterium, the A-layer protein and the o-polysaccharide component of LPS. In the chronically infected fish the humoral response was primarily directed against the A-layer protein.