Doxorubicin (DOX) is a highly potent anti-neoplastic agent, which is widely used in clinical practice; however, its dosage and duration of administration are strictly limited due to dose-related cardiac damage. The present study investigated whether theanine, an amino acid found in green tea leaves, can reduce DOX-induced acute cardiotoxicity by examining changes in serum enzyme activities. The activities of creatine kinase (CK) and lactate dehydrogenase (LDH), biochemical markers of cardiac impairment, in serum were elevated by the administration of 20mg/kg DOX to mice. Elevations in serum enzyme activities were attenuated by concurrent treatment with theanine, and significant suppression was observed at a dose of 10mg/kg, which is comparable to daily intake in the form of supplement. Especially, CK activity was recovered by theanine almost to control levels. Body weight loss caused by DOX was significantly suppressed by theanine treatment. These results suggest that theanine can protect against DOX-induced acute cardiac injury.