ABSTRACT. In this study, we investigated whether ozonated autohemoadministration (OAHA) influences leukocyte populations in cows with clinical inflammatory disease. Eleven cows with inflammatory disease (Inflammatory Group) and three healthy cows (Control Group) were used for this study. The CD4 + /CD8 + ratio in the Inflammatory Group increased significantly compared to that in the Control Group 3 to 4 days after OAHA treatment. In the Inflammatory Group, the number of CD14 + cells decreased gradually after OAHA, but CD14 + levels remained stable in the Control Group. The number of MHC class-II + cells decreased gradually in the Inflammatory Group, but increased gradually in the Control Group, and the difference between the groups was significant on day 14 after OAHA. These findings suggest a possible difference in the activation of immune response after OAHA in infected cows compared to healthy cows. KEY WORDS: dairy cow, leukocyte population, ozonated autohemoadministration.