A mouse monoclonal antibody AK97 (IgM) was established against a new type of glycosphingolipid, SEGLx, isolated from plerocercoids of tapeworm, Spirometra erinaceieuropaei. The chemical structure of SEGLx (Gal beta1-4(Fuc alpha1-3)(Glc beta1-3Gal beta1-ceramide) had been previously characterized. The specificity of AK97 was determined by thin-layer chromatography-immunostaining and enzyme-linked immunosorbent assay. AK97 was found to be directed to SEGLx and GalSEGLx (Gal beta1-4(Fuc alpha1-3)Glc beta1-3(Gal beta1-6)Gal beta1-ceramide) and also showed cross-reactivity with the stage specific embryonic antigen-1 (SSEA-1), the epitope being defined to be the non-reducing terminal trisaccharide sequence. On immunohistochemical examination, AK97 predominantly stained the tegument, the external surfaces of worms which have a brush border-like organization. Based on the immunohistochemical findings for the staining liability as to organic solvents and the results of Western blot analysis of the plerocercoid glycoproteins, it was proved that the antigens in the tapeworm were glycolipids. Considering that the tapeworm is in direct contact with its host's tissue through the tegument, the membrane surface of which is exposed to the external environment, it is suspected that SEGLx and GalSEGLx on the tegument play functionally important roles in the host parasite interaction.