The antigen recognized by MAb YH206 is mainly expressed in adenocarcinomas and is also detected in the sera of cancer patients (Hinoda et al., 1987). This antigen (antigen YH206) was chemically characterized and purified by column chromatography. SDS-PAGE analysis revealed a broad component in the high-molecular-weight range which was clearly detectable by carbohydrate (PAS) but not by protein (silver) stain. Agarose gel electrophoresis and immunoblotting of antigen YH206 indicated that it consists of a high-molecular-weight component (more than 2,000 kDa). Treatment of antigen YH206 with alkali suggested that the antigenic determinant consists of carbohydrate chains of mucin type. Density gradient ultracentrifugation revealed that the activity of antigen YH206 is localized at a density of 1.45 g/ml, suggesting that antigen YH206 is a mucin. Neuraminidase treatment of antigen YH206 indicated that the epitope is cryptic and contains an asialocarbohydrate chain. Once antigen YH206 has been purified by affinity chromatography, neither CA19-9 antigen nor DU-PAN-II antigen can be detected, although they were present at very high levels in the crude ascitic starting material; these last two are representative carbohydrate antigens which are widely used for serodiagnosis to detect adenocarcinomas.