Introduction The objective of this bovine peripheral blood study was a comparative assessment of the phagocytic activity of neutrophils and monocytes and of the intracellular killing capacity of neutrophils from cows given no probiotic and from cows which were administered a probiotic consisting of Saccharomyces cerevisiae, Lactobacillus acidophilus, Lactobacillus plantarum and Rhodopseudomonas palustris. These activity types were compared during different lactation periods. Material and Methods A cohort of 20 pregnant dairy cows was divided into two groups of 10. The experimental group consisted of cows fed a ration supplemented with probiotics, and the control group consisted of cows fed an unsupplemented ration. Blood was drawn six times: 7 days before dry off, 14 days before parturition, and 7, 21, 60 and 90 days postpartum (DPP). The phagocytic activity of neutrophils and monocytes and the oxidative burst activity of neutrophils were determined by flow cytometry in the peripheral blood of all examined cows. Results Phagocytosis testing revealed increased percentages of phagocytic neutrophils and monocytes in the experimental group at 21, 60 and 90 DPP (P-value < 0.01). The mean fluorescence intensity (MFI) values for neutrophils and monocytes were higher on all days of the study (P-value < 0.01). In oxidative burst testing, the percentages of detected neutrophils and their MFI were increased in the experimental group on all days (P-value < 0.01). Conclusion The use of probiotics supported dairy cows’ immunity throughout the whole experiment. Probiotic supplementation may limit the occurrence of infectious diseases in these animals.