Objective: In order to identify a reliable marker for the early detection of muscle injuries in sports, a-actin protein and other markers of muscle damage were studied in sera of uninjured sportspeople and those with skeletal muscle injury. Methods: Blood samples were obtained from 20 sportspeople with skeletal muscle injury and 48 uninjured sportspeople. Immunoassays were performed to determine cardiac troponin I (TnI), troponin T, lactate dehydrogenase and myoglobin concentrations. Western blot and densitometry were used to measure a-actin concentrations. Skeletal muscle damage was diagnosed according to physical examination, MRI findings and the biochemical criterion of a creatine kinase value .500 IU/l (Rosalki method, Beckman Instruments SL, Fullerton, California, USA). Results were also compared with previously obtained data on injured and uninjured non-sportspeople. Results: The mean serum concentration of a-actin was significantly higher in sportspeople with muscle damage (10.49 mg/ml) than in uninjured sportspeople (3.99 mg/ml). Sera from injured sportspeople showed higher levels of a-actin than of troponin or myoglobin. No significant difference in TnI levels was observed between the groups. Conclusions: According to these results, a-actin is a new and reliable marker of skeletal muscle damage in sportspeople which can be used for the detection of muscle injury. Possible cross interference between skeletal and cardiac muscle damage can be discriminated by the combined use of a-actin and TnI. These data suggest that early measurement of a-actin in sportspeople with suspected muscle damage will allow them to receive earlier and more effective treatment and to return sooner to the practice of their sport.