Serum from a dog immunized with blood plasma from a B. gibsoni-infected dog, putatively containing secreted antigens, was used to screen a cDNA expression library. A novel gene encoding BgSA1 was identified from the isolated clones. The serum raised in mice immunized with the recombinant BgSA1 expressed in Escherichia coli could recognize a native parasite protein with a molecular mass of 59 kDa. Comparing with the previously established ELISA with recombinant P50 as antigen, the ELISA with recombinant BgSA1 as the antigen was more sensitive when they were used to detect field samples. Moreover, a sandwich ELISA with anti-BgSA1 antibodies could detect the circulating BgSA1 in a serial blood plasma from a dog experimentally infected with B. gibsoni. These results indicated that BgSA1 could be a useful target for the development of a diagnostic test for the detection of specific antibodies and circulating antigens.