Intraperitoneal infection of mice with larvae of the cestode parasite Taenia crassiceps results in depression of both primary and secondary antibody responses to sheep erythrocytes in vivo. The depression is not associated with a shift in kinetics of the response. Both immunoglobulin M (IgM) and IgG responses are depressed, but IgG is depressed more than IgM in the secondary response. Secondary in vitro sheep erythrocyte responses are consistently depressed in both spleen and mesenteric lymph node cell preparations from infected mice, whereas primary in vitro sheep erythrocyte responses are consistently depressed in mesenteric lymph node cell preparations but not always in spleen cell preparations. These results are consistent with antigenic competition. The cell type or types involved in mediation of the immunological defect in the infected animals remain to be identified.