Cockroach infestations have been linked with allergic diseases such as asthma in humans. Bla g 5, sigma class glutathione S-transferase (GST), is the major cockroach allergen which has the highest IgE response value of all cockroach allergens. Although several cockroach allergens have been identified and cloned, information regarding their B ell and T cell IgE-binding epitopes is limited. In order to analyze the IgE binding epitopes of Bla g 5, full-length and five peptide fragments (A, 1-100 amino acid residue; B, 91-201; Ba, 1-125; Bb, 1-150; Bc, 1-175) were expressed. Twelve (37.5%) of 32 sera from cockroach-sensitized subjects showed positive IgE reactivity to the recombinant Bla g 5 (rBla g 5). Six strong positive sera were selected for the epitope study. Recombinant proteins not containing 176-201 amino acid residues were unable to react to sera from cockroach sensitized individuals, suggesting that this region contains the IgE-binding epitope. Despite strong IgE reactivity to rBla g 5, the pooled serum from 5 cockroach-sensitized patients did not show IgE reactivity to all synthetic peptides consisting of 15 residues covering 161-201 amino acids. These results suggest the possibility that Bla g 5 may have a conformational epitope in the C-terminal region. GST is the important target for the development of vaccines and drugs against allergic diseases because of high cross-reactivity among insect species. This study will aid recombinant allergen research for immunotherapy of cockroach allergens and other insect allergens.