Human liver specific F antigen has been purified from human liver and characterized by immunochemical and physicochemical procedures. Specific antisera were prepared by immunizing guinea pigs with the purified human F antigen. The purified F antigen had a molecular mass of approximately 80000 daltons. Using the immunodiffusion technique, it was shown that human F antigen has at least two antigenic determinants; one is species non-specific and the other human specific. This human specific determinant was also found in the liver of rhesus monkey but not in the liver of the other mammals tested. Using purified human F antigen and antibody against the human specific determinant, F antigen can be quantitated in serum and may prove to be of clinical value.