Toxocara excretory-secretory antigens (TES) were isolated from the culture media of T.canis and T.cati larvae and their O-glycan content was investigated using fast atom bombardment-mass spectrometry (FAB-MS), gas chromatography and electron impact mass spectrometry. The major oligosaccharides released by reductive elimination of T.canis TES glycoproteins were shown to be two, approximately equi-abundant, trisaccharides: 2-O-Me-Fucp(alpha 1----2)-4-O-Me-Galp(beta 1----3)GalNAcitol and 2-O-Me-Fucp(alpha 1----2)-Galp(beta 1----3)GalNAcitol. In contrast T.cati TES O-glycans are predominantly one component, shown by FAB-MS to be a di-O-methylated trisaccharide, which is probably identical to the di-O-methylated trisaccharide from T.canis. The O-methylated trisaccharides are strong candidates for the carbohydrate epitopes recognized by a panel of monoclonal antibodies which exhibit multiple reactivity against TES antigens. This study constitutes the first rigorous characterization of glycans from a parasitic nematode.