The properties of the antigen recognized by monoclonal antibody FH6 have been analyzed. FH6 was originally generated against a glycolipid, i.e. a difucoganglioside isolated from human colonic adenocarcinoma, and specifically reacts with sialyl Lex-i determinant. Several culture supernatants of human carcinoma cell line cells were found to have high levels of FH6-reactive antigen, and PC-9, a human lung carcinoma cell line was used for the analysis. A solid-phase sandwich radioimmunoassay was performed to detect the antigen. The antigenic activity was extractable in 0.6 M PCA or 7% TCA, and was sensitive to mild alkaline treatment and to Pronase digestion. Most of the antigen was eluted in the void volume of a Sepharose CL-2B column, which indicates that its molecular weight is greater than several million. It was eluted from a DEAE-cellulose column at a NaCl concentration in the range of 0.2-0.25 M. The immunoaffinity-purified antigen has a high carbohydrate content of more than 80%. These data indicate that the antigen recognized by FH6 in the culture supernatant of PC-9 is not a glycolipid, but a high molecular weight glycoprotein which could be referred to as a mucin, or a proteoglycan, which contains keratan-sulfate like glycosaminoglycan chains, as judged from the results of the glycosidase treatments.