Three-week-old ICR SPF mice were orally inoculated with one of 5 doses ranging from $2{;} imes{;}10^2{;}to{;}2{;} imes{;}10^6$ oocysts of Crwptosporidium tsuris (strain MCR) per mouse. Oocyst inoculation was directly proportional to the amount of oocysts shed and was inversely proportional to the period required for peals oocyst production and to the prepatent period. Peak oocyst production occurred between fifteen and thirty-one days with a patent period from 61 to 64 days. Three days after all mice stopped shedding oocysts, they were orally challenged with a single dose of $2{;} imes{;}10^6$ oocysts or the same species. Marked seroconversion for IgG antibody accompanied recovery from mice inoculated with $5{;} imes{;}10^5$ oocysts. Mice administered with carrageenan excreted a small number of oocysts for 49.0 days on the average after challenge inoculation (ACI) and control mice for 14.2 days in a dose-independent fashion. Just before challenge infection, phagocytic activity of peritoneal macrophages ($M{phi}$) and the number of peripheral $M{phi}$ were dramatically decreased. Mild challenge infection implies that the immunogenicity of C. nuris (strain MCR) is very strong, despite $M{phi}$ blocker carrageenan administration.