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The purpose of this study was to validate an equine-specific osteocalcin (OC) radioimmunoassay (RIA) for use in donkeys and to establish age-related changes in serum OC concentrations in healthy donkeys. Serial dilutions of donkey serum showed parallelism with standard curves obtained with the equine-specific OC RIA. There was a tight linear regression between donkey serum OC values obtained with the equine specific OC RIA and a commercially available bovine-specific OC RIA. Serum OC levels of 27 healthy donkeys, analysed with the equine-specific OC RIA, showed a tight negative logarithmic regression with age. Least square means and standard error of serum OC values were 67 +/- 10 microg/l in 0.6-4-year-old donkeys (group 1), 32 +/- 9 microg/l in 5-9-year-old donkeys (group 2) and 15 +/- 11 microg/l in > 9-year-old donkeys (group 3). Serum OC values were significantly (p < 0.01) higher in group 1 than in groups 2 and 3. The equine-specific OC RIA may be a useful and practical tool to assess bone metabolism and skeletal diseases in donkeys.