Taeniid cestodes infect humans and livestock, causing considerable morbidity and mortality, as well as economic loss. Substantial progress has been made toward the production of recombinant vaccines against cysticercosis in livestock animals. Further development of these vaccines would be aided if a reliable in vitro test were available to measure host-protective immune responses in vaccinated animals. Here, we describe in vitro oncosphere-killing assays for the quantification of host-protective serum antibodies against Taenia pisiformis, Taenia ovis, Taenia saginata, and Taenia solium in rabbits, sheep, cattle, and pigs, respectively. Activated oncospheres of T. pisiformis, T. ovis, T. saginata, and T. solium were incubated in vitro in culture medium, test serum, and a source of complement, and oncosphere killing was assessed after 10 days of culture. In vitro oncosphere killing reflected the presence of specific antibody, and the oncosphere-killing assay typically indicated immunity to the homologous parasite that had been determined in vivo. This study describes the first reliable oncosphere-killing assays for T. pisiformis, T. avis, T. saginata, and T. solium. These assays will be used for further research into the optimization of recombinant vaccines against cysticercosis. © American Society of Parasitologists 2006.