SummaryMice infected with Sarcocystis marts showed a significant reduction in plaque-forming cells (PFC) and delayed-type hypersensitivity (DHS) responses to an unrelated protein antigen, bovine y-globulin, when compared with uninfected controls. This immunosuppression was observed only when infection preceded immunization or when mice were immunized concurrently with infection, suggesting that suppression induced by murine sarcocystosis affected the induction and/or the differentiation of antigen-sensitive immunocytes. The immunosuppression lasted for 5 weeks, the period of this study, and affected cell-mediated responses more than antibody responses. The secondary PFC and DHS responses of mice immunized 14 days after infection and rc-immunized 21 days later were also significantly lower than those of uninfected controls, whereas the secondary PFC and DHS responses of mice primed before infection were unimpaired. This indicated that S. muris infection affects only the induction but not the expression of immune memory.