Lysozyrne activity was studied in vanous species of fish (13 mild species and cultured rainbow trout Salmo gairdnen and Atlantic salmon Salmo salar). Marked interspecies variation prevailed, most strikingly between rainbow trout and Atlantic salmon; lysozyme activity in the former was at least 20 times greater than in the latter. Tissue distribution was studied in more depth in rainbow trout. Kidneys appeared to have the highest lysozyme levels, followed in descending order by alimentary tract, spleen, slan mucus, serum, gills, liver and muscle. This pattern is compatible with the assumption that lysozymes play an important role in body defence. Using gel filtration chromatography, 2 lysozyrne-like variants with molecular weights (MW) of ca 14 500 (high activity) and 23 000 daltons (low activity) were found in rainbow trout kidn.ey. Only low MW activities (between 13 000 and 12 000 daltons) were detected in Atlanlc salmon. Relevant future research approaches, as well as possible or speculative applications of lysozymes in disease control, are discussed.