Ovomucoid, a major allergen in hen's egg white, consists of three tandem domains. The third domain (DIII) cDNA was sublconed into pGEMT-vector and the resultant plasmid (PGEMDIII) was inserted into a pGEM-4T-2 glutathione-S-transferase (GST) fusion vector. The GST-DIII fusion protein was expressed in Escherichia coli. The 56-residue fragment corresponding to DIII (Leu(131)-Cys(186)) was liberated using cyanogen bromide to cleave off the G:ST that had been hydrolized with thrombin, which left an additional peptide at the terminus of the recombinant protein. Measurement of circular dichroism spectra indicated that the recombinant third domain (DIII*) had a structure that was slightly less compact than that of the native form. Immunoblot analysis showed that the human IgE binding activity of DIII* was identical to that of native DIII, while its activity was significantly increased to IgE antibodies from egg-allergic patients when tested with an enzyme-linked immunosorbent assay. These results indicate that recombinant DIII* has similar sequential epitopes, but may have more predominant conformational epitopes than native analogues. This might have important implications in egg-allergic reactions.