The nematode parasite Toxocara canis is found in all dog populations and poses a poorly defined health hazard to humans. We have studied excretory-secretory antigen (ES) and surface antigens of the infective larval stage which is tissue-invasive in mammalian hosts. Antigens were probed with a panel of eight monoclonal antibodies raised in mice to whole ES. Six of eight antibodies reacted with periodate-sensitive carbohydrate epitopes on ES molecules, and the remaining two (Tcn-3 and Tcn-6) recognized either peptide or periodate-resistant sugar determinants. By immunoprecipitation and immunoblotting, the anti-carbohydrate monoclonals each reacted with several distinct ES molecules, known from previously published work to possess contrasting biochemical properties. Tcn-3 and -6 were directed predominantly against 32,000 and 120,000 m.w. molecules, respectively. Iodinated surface antigens of similar m.w. were precipitated by each antibody after detergent solubilization, but only two clones (Tcn-2 and -8) were able to bind exposed sites on the epicuticle of intact Toxocara larvae. Significantly, these antibodies do not bind to newly hatched larvae, and their target antigens are poorly expressed until the second day of in vitro cultivation. The specificities of the monoclonals were further studied by cold antibody inhibition of radiolabeled monoclonal binding, and by a matrix of two-site binding assays. These data show that Tcn-2, -4, -5, and -8 recognize a related group of repetitive carbohydrate epitopes, whereas Tcn-1, -6, and -7 bind discrete determinants on the same molecules. These studies are being continued to define further the structure of antigenic Toxocara carbohydrates and to compare the diagnostic utility of carbohydrate and peptide antigens.