Introduction: The aim of the study was to compare selected leukocyte subpopulations and the serum amyloid A (SAA) concentration in the peripheral blood of cows at different stages of lactation. The blood of cows receiving a probiotic as a dietary supplement was compared with the blood of cows not receiving it. Material and methods: The research was conducted on 20 pregnant dairy cows randomly divided into two groups of 10 cows each. The experimental group consisted of cows given the probiotic as a feed supplement. The control group consisted of cows that were fed without supplementation. Blood was drawn six times for testing: 7 days before drying; 14 days before parturition; and 7, 21, 60 and 90 days postpartum. Leukocyte immunophenotyping was performed by flow cytometry. Results: The blood of cows administered the probiotic revealed an increased percentage of forkhead box protein 3 (Foxp3)+, T CD4+ and B CD25+ lymphocytes and β2 CD18+ and αM CD11b+ integrins, and persistently low SAA levels at all time points. Conclusion: The activity of the immune system in cows receiving the probiotic was higher than in control cows. However, the stabilisation of the immune system of the supplemented cows may be indicated by the persistence of a low level of SAA throughout the experiment. Therefore, it can be assumed that the immune system of cows treated with the probiotic more easily adapts to changes in conditions in particular lactation periods and that these cows become more resistant to infectious diseases. Keywords: dairy cows; integrins; leukocyte subpopulations; probiotics; serum amyloid A.