The adaptation and immunogenisity of Cryptosporidium parvum isolated from Siberian chipmunks (SC1 strain) in immunocompetent (ICR) mice were examined. The oocysts were received to the severe combined immunodeficiency (SCID) mice by repeated passage. The oocysts collected from the 18th SCID mice were inoculated to 5 ICR mice. The mice began to shed oocysts from 6 days after inoculation, the patency was 5 days, and the maximum oocysts per gram of feces (OPG) value was 10(4). The maximum of OPG value was gradually increased by successive passage, and finally that in the 22nd mice reached 10(6) (patency: 11 days). It is considered that these results indicate completion of their adaptation to ICR mice. To examine the immunogenicity of C. parvum to ICR mice, 8 groups of 5 mice each were inoculated with 1.3 × 10(6) oocysts of SC1 strain, which were collected after adaptation to SCID mice. All groups shed oocysts from 6th day, and their patency was from 8 to 12 days. On the 21st day after the primary infection, these mice were challenged with 1.3 × 10(6) oocysts. No oocysts shed from any groups, although 2 control groups shed oocysts from the 6th day, and their OPG values were more than 10(6). These results suggest that this strain has strong immunogenicity against ICR mice. Therefore, the immunological healthy mice were considered a useful experimental model to investigate immunological and drug treatments in the strain of C. parvum.