A monoclonal antibody CSLEX1 which reacts with sialosyl Le^x but not with sialosyl Le^a has been produced. The CSLEX1 antigen has a tissue distribution similar to that of Le^x, appearing characteristically in the proximal tubules of the kidney and on granulocytes. It is tumor associated in that 14 of 34 (41%) of tumor lines tested reacted with the CSLEX1 antibody, and 50 of 74 (68%) of tumor tissues tested reacted with the antibody. Loss of immunoperoxidase staining of tissues after neuraminidase treatment showed that the antibody is reacting to sialyl derivatives. The antibody reacted in solid-phase radioimmunoassay to sialosyilactofucopentaosyl(III)ceramide and sialosyldifucosylgan-glioside (6B). These results indicate that the CSLEX1 epitope has the following structure: This structure had not previously been known to be tumor associated.