To isolate Babesia equi genes encoding immunodominant proteins, a cDNA expression library prepared from B. equi mRNA was immunoscreened with B. equi-infected horse serum. Eighteen positive cDNA clones were obtained, and the clone that showed the strongest immunoreactivity, designated Be82, was further characterized. The Be82 gene consisted of 1,953 bp and contained a partial open reading frame lacking the 5'-terminal sequence. As shown by Western blot analyses, immune sera from mice intraperitoneally injected with the Be82 gene product recognized the 82- and 52-kDa proteins of B. equi but not those of Babesia caballi. The glutathione S-transferase fusion protein expressed in Escherichia coli that was purified and used as the antigen in the enzyme-linked immunosorbent assay reacted specifically with B. equi-infected horse sera. These results suggest that the Be82 gene product is a potential diagnostic antigen candidate in the detection of B. equi infection in horses that will be useful both in the performance of epidemiological studies and in the granting of quarantine passes.