To determine the role of cytokines and a chemokine receptor in the susceptibility to, and outcome of, infection, 4 different knockout mice (IL-4, IL-10, IL-12, and CCR5) were infected with Cryptosporidium parvum and monitored for infection intensity by collection of fecal pellets from individual mice. Because adult immunocompetent mice are refractory to infection, wild-type mice on the same background as the knockout mice (C57BL/6) were used as a negative control. No infection was detected over a 4-wk time period in IL-4, IL-10, and CCR5 knockout mice inoculated with 10⁶ oocysts. IL-12 knockout mice inoculated with as little as 100 oocysts shed up to 10,000 oocysts/100 μl of feces on the peak infection day (day 8) and were able to fully recover by 2 wk after infection. IL-12 is an important inducer of IFN-γ, which probably accounted for susceptibility to infection. Previous studies using IFN-γ knockout mice have shown strain-related differences in infection intensity and outcome, with increased parasite loads and decreased survival among IFN-γ knockout mice on a C57BL/6 background compared with those on a BALB/c background. Similar results were observed in IL-12 knockout mice on a BALB/c background, which exhibited little or no infection, despite higher levels of inoculation (10⁶ oocysts/mouse).