Abstract: A new protein, named ACA1, has been isolated and purified from the medical fungi Antrodia camphorata using the technique of anion-exchange chromatography. ACA1, a glycoprotein with a molecular mass of 29 kDa, has a pI value of pH 5.3 and contains 118 amino acids in its peptide moiety. In addition, ACA1 contains methionine, half-cystine and histidine residues, which are not existent in FIP-fve and Ling Zhi-8. ACA1 is not able to agglutinate red blood cells from human and mouse. Moreover, ACA1 possesses immunomodulatory activities, which are demonstrated by their stimulatory activity toward RAW 264.7 macrophages and mouse splenocytes. ACA1 can directly enhance the production of tumor necrosis factor-alpha and nitric oxide by RAW 264.7 macrophages, and induce cell proliferation and interferon-gamma secretion by mouse splenocytes.