This study investigated the differences imposed by the extra set of chromosomes on the immune system of a triploid animal. Various haematological and immunological parameters in diploid and triploid sibling Atlantic salmon (Salmo salar) were compared. Haematocrits were usually the same in diploid and triploid siblings. Erythrocytes and leucocytes were larger and fewer in number in triploid siblings. However, they were not larger or fewer in number by a factor of one third. The proportions of the different leucocyte populations did not differ between diploids and triploids. This was the case for 3 families of diploids and triploids. Non-specific, humoral factors were not consistently different between diploids and triploids. From an investigation of 3 families it was found that there may be family influences on how diploids and triploids compare. A study of the kinetics of responsiveness of non-specific, humoral factors showed that triploids did differ from diploids in the speed of the immune reaction or in the rate of recovery. However, the profile of the differences between diploids and triploids varied between parameters. The rate of increasing agglutinating titres in response to treatment with a commercial vaccine did not differ between diploids and triploids. However, triploids did have a higher average titre after 24 weeks in comparison to diploids. Non-specific, cellular parameters did differ between sibling diploids and triploids. Triploid kidney macrophages had an increased phagocytic capacity but the proportion of phagocytic cells did not differ. Intracellular respiratory burst activity was not different between diploid and triploid kidney macrophages. Diploids and triploids did not respond differently to predator stress and there was no difference in susceptibility to Aeromonas salmonicida ssp. salmonicida challenge. From this study there was no evidence of consistent differences in immmunocompetence or disease susceptibility of diploid or triploid Atlantic salmon.