Cultivated hydrobionts under intensive aquaculture conditions are exposed to stress factors associated with artificial habitats, increased planting density, periodic fishing, etc. At the same time, they experience a decrease in resistance and resilience and an increase in morbidity. A good solution to this problem is the use of immunomodulators. This article presents the results of a study of the effect on fish of the immunomodulator Roncoleukin, which is a dosage form of recombinant human interleukin‑2 (IL-2) isolated and purified from yeast cells (Saccharomyces cerevisiae). The subject of the study was the three-year-old Arctic char (Salvelinus alpinus). The evaluation was based on morphometric, hematological (erythro- and leukogram) and cytochemical (lysosomal cation test) indicators. The coefficient of variation of the size and weight parameters of the treated fish was low compared to the control group. This indicates uniform growth of fish fed Roncoleukin, which is a great advantage in fish farming and provides more efficient use of feed. Blood for analysis was taken from the tail vein of the fish in vivo. The results showed a positive effect of the drug on the activation of erythropoiesis and leukopoiesis. There was a significant increase in the proportion of mature segmented neutrophils (the main blood microphages belonging to the non-specific link of cellular immunity) in the leukocyte formula due to a decrease in lymphocytes (specific immunity). An increase in the content of non-enzymatic lysosomal cationic proteins (defensins) in blood neutrophils can indicate a high level of cellular protection of microphages (neutrophils). The results of the study showed a positive effect of the drug Roncoleukin, which can be recommended for use in the ferming of Arctic char.