Abstract: The present invention provides a method for identifying a biomarker for diagnosis of lymphoma in a canine subject. The method comprises the following steps: (i) providing serum samples from canine subjects with lymphoma (lymphoma samples); (ii) providing serum samples from canine subjects free from lymphoma (control samples); (iii) fractionating the protein components in the serum samples provided in steps (i) and (ii) using anion exchange chromatography; (iv) further purifying proteins from the fractionated samples produced in step (iii) by contacting the proteins therein with a SELDI protein chip comprising a cation exchange surface; (v) characterising the proteins adhered to the cation exchange surface of the SELDI protein chip in step (iv) using mass spectrometry; and (vi) performing a classification and regression tree (CART) analysis to identify proteins capable of acting as biomarkers, either alone or in combination with other proteins.