Three-day-old 105 Ross race female broiler chicks were used. The chickens were divided into 5 groups comprising 1 control and 4 experimental groups. Animals in the control group (group 1) were provided with normal drinking water, whereas the experimental groups, namely, group 2. group 3, group 4 and group 5 were administered with tilmicosin at doses of 20 ppm, 40 ppm, 80 ppm and 160 ppm (approximately, 5 mg/kg/b.w./d, 10 mg/kg/b.w./d, 20 mg/kg/b.w./d, and 40 mg/kg/b.w./d), respectively, via drinking water for a period of 3 d. Seven animals were euthanized from each group, on the 1(st) d (first period), 4(th) d (second period) and 7(th) d (third period) of the experiment for the assessment of blood malondialdehyde (MDA) levels and catalase (CAT) activity. Statistically significant differences in comparison with the control group were observed in plasma MDA levels of all experimental groups in the first period, groups 2 and 5 in the second period, and group 4 in the third period. With regards to CAT activity, a statistically significant decrease was found in all experimental groups only in the first period.