We investigated the response to challenge infection with Cryptosporidium parvum oocysts in immunosuppressed C57BL/6N mice.In the primary infection, fecal oocyst shedding and parasite colonization were greater in immunosuppressed mice than in nonimmunosuppressed mice.Compared with primary infection, challenge infection with C.parvum didn 't show any oocyst shedding and parasite colonization.Especially, oocyst shedding and parasite colonization from the mice infected with heat- killed oocysts were not detected.After challenge infection with C.parvum oocysts, however, these mice were shedding small numbers of oocysts and parasite colonization. Except normal control and uninfected groups, the antibody titers of other groups appear similar.Based on the fecal oocyst shedding, parasite colonization of ilea, and antibody titers in the mice, these results suggest that the resistance to challenge infection with C.parvum in immunosuppressed C57BL/6N mice has increased.