The objective of this study was to evaluate and compare the antioxidant response and the products of oxidative damage analysed by various assays in clinical and subclinical canine monocytic ehrlichiosis (CME). For this purpose, four assays to measure the total serum antioxidant capacity (TAC), such as the cupric reducing antioxidant capacity (CUPRAC), ferric reducing ability of plasma (FRAP), trolox equivalent antioxidant capacity (TEAC) using acidic medium (TEAC(A)), and the TEAC using the horseradish peroxidase (TEAC(H)) were used. In addition, the serum thiol concentrations were analysed. Reactive oxygen species (ROS), thiobarbituric acid reactive substances (TBARS), and ferrous oxidation-xylenol orange (FOX) were measured to determine the concentrations of free radical and the products of oxidative damage as result of the disease. All antioxidant markers were significantly lower in the dogs on clinical ehrlichiosis when compared with healthy dogs; however only the CUPRAC, FRAP and thiol were significantly lower in subclinical CME compared with healthy dogs. TBARS and FOX showed no significant differences between dogs with CME and healthy dogs; however, a significant increased ROS concentration was observed in dogs with clinical and subclinical CME when compared with healthy dogs. Results showed that in CME there is a state of oxidative stress with significant changes in markers of antioxidant defence and in concentrations of free radicals. However, the detection of these changes would depend of the assay used.