Passive immunization of mice with a monoclonal IgE antibody to Schistosoma japonicum (SJ18 epsilon. 1) induces significant protection to a challenge infection and the target molecule of SJ18 epsilon. 1 is paramyosin. In the present study, we demonstrate the B cell epitope of paramyosin recognized by SJ18 epsilon. 1 by using a series of deletion mutants expressed in Escherichia coli. SJ18 epsilon 1 reacted with the recombinant paramyosin containing 113 amino acids (Glu301. Ala413) but not with a shorter peptide (Glu301-Asp343). Further epitope mapping carried out by a multi-pin system using heptameric peptides synthesized sequentially from 71 amino acids of paramyosin (Asp343-Ala413) demonstrated significant binding of SJ18 epsilon. 1 to the sequence, 359Ile-Arg-Arg-Ala362. Replacement set analysis of the pentameric peptide, 358Leu-Ile-Arg-Arg-Ala362, revealed that replacement of each residue with a hydrophobic or hydrophilic amino acid did not inhibit binding of SJ18 epsilon. 1, whereas replacement of positively charged Arg. or hydrophobic Ala with a negatively charged amino acid, Glu, showed reduction in binding of the antibody. Moreover, replacement of each amino acid including Arg with a positively charged amino acid, Lys, resulted in a drastic loss of the binding, indicating that binding of the antibody was markedly affected by the change of charges of the peptide as well as by the conformational alteration. The target epitope of SJ18 epsilon. 1 was common among paramyosins of S. mansoni, Taenia solium and Echinococcus granulosus but not among nematode paramyosins, suggesting that the epitope is specific for platyhelminthes.