Normal rats were given intravenous doses of either immune serum or immunoglobulin fractions 24 hr before oral challenge with 1,000 eggs of Taenia taeniaeformis. Total eosinophil counts per mm3 were performed for 3 days prior to and 6 days after challenge. Sensitized rats usually showed sharp peaks of eosinophilia 2 to 6 days after this dose. The pattern of the eosinophilic response was similar to that which occurs after challenge of immune infected rats. The differences in peripheral eosinophil levels in passively immunized and normal rats were statistically significant. Immunoglobulin fractions containing protective IgG2alpha were most effective, but a fraction containing reaginic antibody activity also sensitized rats to give secondary eosinophilic responses. The findings are discussed in relation to the probable contribution of antigen-antibody reactions to the production of secondary eosinophilic responses in experimental cysticercosis.