Severe combined immunodeficient (SCID) mice were experimentally infected with Cryptosporidium parvum. Adoptive transfer of BALB/c thymocytes, spleen and bone marrow cells resulted in functional immunologic reconstitution followed by complete eradication of the cryptosporidial infection. Additional SCID mice were injected with human blood peripheral blood lymphocytes and were subsequently infected with C. parvum. The latter mice (SCID-hu-PBL) were at least partially reconstituted with human lymphoid tissues, as evidenced by flow cytometric identification of human cell populations in the SCID mouse spleens and the response of these cells to the T-cell mitogen phytohemagglutinin. The SCID-hu-PBL mice did not resolve the cryptosporidial infections, although a transient reduction in parasitemia was noted 4-6 wk post-reconstitution.