The purpose of this study was to quan ti tate the populations o f lymphocytes in the s pleens and intestines of normal and immunosuppressed adult C57BL/6N mice that were noninfected or infected with Cryptosporidi um parvum. This was accomplished by using the following methodologies: immunohistochemistry, ELISA-spot assay, and flow cytometry. Mice in groups 1 and 2 were immunosuppressed, but only group 2 was in fected. Mice in group 3 were only infected, whereas group 4 served as the normal control . Mice were immunosuppressed with dexamethasone (DEXI at a dosage of 125~g / mouse/day. Infected mice received 106 oocysts per os . The numbers o f lymphocytes were monitored from day 0 to day 18 postinfect i on. Flow cytometry using antibodies directed against CD4+ and CDS+ T cells (helper and cytotoxic, respectively) and B cells (expressing IgG, IgM, and IgA receptors) revealed that c. parvum did not evoke an alteration in the phenotypic profile of lymphocytes within spleens or Peyer's patches (PP) of mice in groups 2 and 3 that was statist i cally different from groups 1 and 4. Immunosuppressed mice (groups 1 and 2) had significantly fewer lymphocytes (bearing CD4+, IgG, IgM, and IgA receptors) within the spleen when compared with mice in groups 3 and 4 (P