Researching how the immune system reacts to stimuli such as blood transfusion of alloantigens on models of different species of laboratory animals, in particular rabbits, is important for understanding the mechanisms through which post-transfusion reactions develop. In this study, we identified the absolute and relative contents of white blood cells in blood of the recipient rabbits, and also their subpopulations after allogenic transfusion of packed red blood cells (pRBCs). Allogeneic transfusion of pRBCs without leukoreduction was conducted once, in the calculation of 5.5 mL/kg of body mass of the experimental animals. The material for the studies were samples of blood serum, collected from 5 rabbits on the 3rd, 7th, and 23rd days after pRBC administration. Allogeneic transfusion of pRBCs to the recipient rabbits caused development of leukocytosis in their blood: compared with the initial condition, the absolute content of leukocytes in the blood of the experimental animals increased 27.0% on the 3rd day after transfusion, equaling 8.94 109/L; 40.1% on the 7th day, accounting for 9.86 109/L; and 34.1% on the 23rd day, measuring 9.44 109/L. Leukocytosis resulted from increased content of granulocytes in the blood compared with the initial state: the absolute content of granulocytes surged 66.9% on the 3rd day after transfusion, measuring 4.54 109/L, 2.21-fold on the 7th day, equaling 6.02 109/L, and 1.87-fold on the 23rd day, equaling 5.08 109/L. Allogeneic transfusion of pRBCs to the recipient rabbits caused decrease in the relative content of Т-lymphocytes and increase in the content of В-lymphocytes in their blood. Compared with the initial condition, the relative content of Т-lymphocytes in the blood of the experimental animals declined 20.6% on the 3rd day of the study, accounting for 58.0%; 30.1% on the 7th day, equaling 51.0%; and 25.8% on the 23rd day, equaling 54.2%. Compared with the initial state, the relative content of В-lymphocytes spiked 94.7% on the 3rd day of the study, measuring 22.2%, 2.23-fold on the 7th day, accounting for 25.4%, and 2.51-fold on the 23rd day, equaling 28.6%.