Giardia lamblia, a protozoan causing diarrheal outbreaks, is one of the main pathogens monitored in developed countries. Immunoscreening of G. lamblia expression library using the monoclonal antibodies (mAb) against G. lamblia, identified a subset of antigenic proteins in this protozoan, which are proteins belonging to GHSP (Giardia head-stalk protein), GHSP115, GHSP138, and GHSP180. In order to map the epitope region of GHSP115, the corresponding open reading frame was dissected into three parts and expressed as recombinant proteins with histidine tags. Western blot analysis of these recombinant proteins with mAbs reacting with GHSP115 indicated that one-third of the C-terminus of GHSP115 showed immunoreactivity with the mAb. Intracellular location of GHSP115 was examined both in trophozoites and encysting cells of G. lamblia by an immunofluorescence assay, indicating that location of GHSP115 varies during encystation. These results suggest that GHSP115 is an abundant and antigenic protein, which is differentially localized during life cycle of G. lamblia.