The epitope recognized by a mouse monoclonal antibody (MAb) to the crystalline surface layer protein (SLP) of Rickettsia typhi, SRT10, is mapped to ten amino acid residues (SRTag, TFIGAIATDT). The oligonucleotide sequence covering the epitope recognized by SRT10 is inserted to a mammalian expression vector together with multiple cloning sites. When the SRTag is fused in frame to the coding region of protein gene and expressed in bacteria or mammalian cells, the MAb SRT10 can detect the tagged protein by immunoblotting, immunocytochemistry, and immunoprecipitation.