The importance of B cells in host resistance to, and recovery from, Cryptosporidium parvum infection was examined in gene-targeted B cell-deficient (muMT-/-) mice. Neonatal muMT-/- mice infected with C. parvum at 5 days of age completely cleared the infection by day 20 PI. The kinetics of infection and clearance were similar to those seen with age-matched C57BL/6 control mice. Furthermore, B cells were not required to clear existing C. parvum infection in adult mice. Reconstitution of persistently infected Rag-1-/- adult mice with spleen cells from muMT-/- donor mice resulted in significant reduction of infection, as in the results seen with spleen cells from C57BL6 donors. These findings indicate clearly that B cells are not essential for host resistance to, and recovery from, C. parvum infection in mice.