Three monoclonal antibodies against Theileria sergenti were prepared and used in indirect fluorescent antibody staining procedure to determine the percentages of infected erythrocytes in a calf under parasitemia for 21 days. The percentages detected by the monoclonal antibodies were slightly lower than those by bovine immune serum or Giemsa stain. A blocking test showed that the three monoclonal antibodies recognize the same antigenic determinant of T.sergenti. Another blocking test also indicated that the monoclonal antibodies recognize one of the antigenic determinants on the organism detected by polyclonal antisera. However, the shapes of T.sergenti stained by the monoclonal antibodies were similar to that stained by Giemsa stain. The immunoglobulin subclass of these monoclonal antibodies was IgG₁, κ light chain.