The paper presents an assessment of the phagocytic ability of blood cells of Nile tilapia Oreochromis niloticus L. under the hormonal induction of stress with range of duration. It has been revealed that Staphylococcus aureus is phagocytosed by leukocytes, platelets, and erythrocytes of tilapia blood in direct smears. The phagocytic capacity of the cells has been quantified. It has been found that long-term exposure to stress has an immunosuppressive effect on the phagocytic activity of leukocytes and an immunostimulating effect on the phagocytic activity of erythrocytes (60-minute incubation) by the end of the experiment in comparison with other groups. Short-term exposure to stress increases the activity of leukocytes and decreases the activity of erythrocytes (60- minute incubation) for 7 days after exposure.