A soluble antigen isolated from Eimeria stiedai merozoites with a molecular mass of 49 kDa was detected in the bile of infected rabbits. Rabbits immunized with the antigen shed a lower number of oocysts than did nonimmunized rabbits postchallenge (p.c.). The immunized rabbits showed a marked and transient increase of alanine-aminotransferase (ALT) activity on day 8 p.c. The blood indocyanine green (ICG) clearance and r-glutamyltransferase (GGT) activity showed no change throughout the experiment However, nonimmunized rabbits showed a gradual increase of ALT and GGT in the plasma and a delay of ICG p.c. Many merozoites were observed in the biliary ducts of the nonimmunized rabbits on day 8 p.c. using standard histology. In contrast, in the immunized rabbits, many inflammatory cells were observed around the biliary ducts, but there were few parasites in the tissue. These results suggest that the 49-kDa soluble protein antigen detected in the bile of the infected rabbits was a merozoite-specific antigen, and the immune reaction to the antigen may induce protective effects against the infection.