Neutrophils collected from the mammary glands of uninfected sheep or from sheep infected with Taenia hydatigena, attached to and killed T. hydatigena oncospheres in vitro in the presence of serum from infected sheep. Infected sheep serum alone was not deleterious to the parasite in vitro. Fc receptors for antibody were detected on both normal and immune neutrophils; they were present at a greater density on the latter. Immune neutrophils were more reactive towards oncospheres than normal neutrophils and formed extensive capsules around the parasite. Fc receptors were not detected on oncospheres. It is hypothesised that neutrophils may kill the parasite by producing hydrogen peroxide and the superoxide anion, both of which are toxic to a variety of cell types and protozoa. The function of antibody may be to facilitate attachment of neutrophils to oncospheres by way of their Fc receptors.