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Streptococcus agalactiae,Streptococcus dysgalactiae and Streptococcus uberis are three common pathogens causing bovine mastitis.The GapC of Streptococcus is a highly conserved surface layer protein that is responsible for inducing protective immune response in mice.To identify protective epitope of GapC,a monoclonal antibody (mAb1F2) against the recombinant GapC of S.dysgalactiae was generated by screening a phage-displayed random 12-peptide library.After three rounds of biopanning,25 phage clones were randomly selected and their specificity to the mAb was verified by sandwich ELISA.Their amino acid of 13 among total 25 phage colones were sequenced and deduced.It was found that their amino aicd sequences can match amino acids 30TRINDLT36 of GapC.It showed that mAb1F2 could react with the motif 30TRINDLT36 fused GST in Western bloting.So it proposed that the motif 30TRINDLT36 represented a linear B-cell epitope of the GapC.A single replacement of amino acid for the epitope 30TRINDLT36 demonstrated that residues R31A,I32A,N33A,D34A and L35A constitute the core of the epitope.The high homology was found after alignment of the epitope sequences from different streptococcus species.The identified epitope would be useful in development of epitope-based marker vaccines against Streptococcus infection.