Persistent infection was established in SCID mice given 10(7) Cryptosporidium parvum oocysts. Nine groups of infected SCID mice were inoculated with 10(6), 10(5), or 10(4) total spleen cells, CD8-depleted spleen cells, or CD4-depleted spleen cells from naive BALB/c donors. Infection was significantly reduced in all treatment groups. The most profound effect occurred with spleen cell preparations containing CD4 T lymphocytes but depleted of CD8 T lymphocytes.