The aim of this study was to follow the effects of different doses of tilmicosin on malondialdehyde and reduced glutathione levels of heart and liver, and on selected haematological indices. Forty male Balb/C mice were used throughout the experiment. They were divided into four groups (n = 10), and injected subcutaneously as follows: Group 1 (control), with isotonic saline solution, Group 2 with 25 mg/kg body weight of tilmicosin, Group 3 with 50 mg/kg, and Group 4 with 75 mg/kg of tilmicosin in single injections. After three days, plasma, cardiac and hepatic malondialdehyde and reduced glutathione levels were measured with spectrophotometer. Red blood cell, white blood cell, platelet, haemoglobin, packed cell volume and percentage of leukocytes were also determined. At the end of the experiment, tilmicosin did not cause any statistically significant (P > 0.05) changes in haematological parameters such as red blood cells, white blood cells, platelet, haemoglobin, packed cell volume and percentage of leukocytes. Hepatic malondialdehyde and reduced glutathione levels increased (P < 0.05) only at the highest dose of tilmicosin. The results indicate that tilmicosin did not cause lipid peroxidation in the heart.