Recognition of specific carbohydrate structures, which occur commonly on the surfaces of invading pathogens, is thought to elicit internal defense mechanisms in invertebrates. To investigate the nature of carbohydrates that evoke a defensive response in hemocytes of the gastropod Biomphalaria glabrata, we tested eight different carbohydrates, conjugated to bovine serum albumin (BSA), for generation of reactive oxygen species (ROS). Six of the carbohydrate moieties tested are thought to be present on the S. mansoni sporocyst surface (mannose, galactose, fucose, N-acetyl-glucosamine, N-acetyl-galactosamine, and lactose); the other two carbohydrates tested were glucose and melibiose. ROS generation was measured using the fluorescent probe - 2',7'-dichlorofluorescein-diacetate (DCFH-DA). Hemocytes were derived from two different strains of B. glabrata: one of the strains used (MO) is susceptible to infection by the trematode Schistosoma mansoni (PR-1 strain), while the other snail strain (13-16-R1) is resistant to infection with PR-1. Three of the BSA-carbohydrate conjugates (BSA-galactose, BSA-mannose, and BSA-fucose), stimulated generation of reactive oxygen species in the molluscan hemocytes. The responses of the hemocytes were similar whether they were derived from susceptible or resistant snails. If the carbohydrate structures we found, to stimulate ROS generation are involved in parasite recognition, our results suggest that parasite killing may involve either qualitative differences in production of reactive oxygen species, or additional factors.