The protective effect of L-carnitine was investigated against tilmicosin-induced cardiotoxic effects including blood creatine kinase (CK), CK-MB, total sialic acid as well as the alterations in glutathione and malondialdehyde concentrations in mice. Thirty-two Balb/C mice were divided into four groups including group I (control), group 2 (L-carnitine, s.c., 500 mg/kg for 5 days), group 3 (tilmicosin, s.c., single dose of 75 mg/kg) and group 4 (L-carnitine plus tilmicosin). Serum CK, CK-MB and malondialdehyde (MDA) levels were significantly (P < 0.05) higher in group 3 compared with those of other groups. Total sialic acid level in group 3 was found to be significantly (P < 0.05) higher than that in groups I and 2, as well. Contrary to these results, glutathione level in group 3 was found to be significantly (P < 0.05) lower than that in groups 1 and 2. In group 4, serum CK, CK-MB, MDA and total sialic acid levels were found to be significantly (P < 0.05) lower than those in group 3. These results suggest that tilmicosin is cardiotoxic in mice as evidenced by higher total sialic acid, CK and CK-MB. In addition, tilmicosin caused the decrease in glutathione and increase in MDA levels. However, administration of L-carnitine could ameliorate these adverse toxic effects of tilmicosin in mice.