Abstract A new monoclonal antibody (IB9) was prepared by hybridoma technique directed specifically to sialosylα2→6galactosyl residue. With this reagent, accumulation of two major gangliosides in human colonic and liver adenocarcinoma has been detected, and these gangliosides were isolated and characterized as structures A and B (below). Another ganglioside with a ceramide nonasaccharide structure, which reacted to anti-X-hapten antibody after desialylation, was also isolated and partially characterized. 1. (A) NeuAcα2→6Galβ1→4GlcNAcβ1→3Galβ1→4Glcβ1→1Cer 2. (B) NeuAcα2→6Galβ1→4GlcNAcβ1→3Galβ1→4GlcNAcβ1→3Galβ1→4Glcβ1→1Cer These gangliosides were absent or present in very small quantity in normal tissue and may represent human cancer-associated markers.