Production of circulating specific antibodies to the lung fluke (Paragonimus westermani) by its host is well known and used in various kinds of immunodiagnostic methods. However, it has not been well documented which compartments (or structures) of the lung fluke are most responsible for the production of specific antibodies. The present immunohistochemical study was undertaken to demonstrate the antigenicity of each body compartment of P. westermani such as suckers, tegument, spines, vitelline glands, intestine, reproductive organs (male and female), and eggs. Indirect immunoperoxidase (IP) stain technique was applied, using formalin-fixed, paraffin-embedded lung tissues of P. westermani-infected cats sectioned in 4 µm thickness as the antigen and cat antisera (11-20 weeks of infection) as the primary antibody. Peroxidase-conjugated goat anti-cat IgG was used as the secondary antibody and diaminobenzidine (DAB) as the coloring agent. Strong yellow or yellowish brown staining was regarded positive. The primary and secondary antibody dilutions were made at 1:500-1:2,000 and 1:200-1:500 respectively, and IP stain was repeated 10 times for each dilution. A consistent result obtained was that the intestinal epithelial border, intestinal content, vitelline glands, and eggs scattered around the worm capsule showed strong positive staining, while uterine eggs and some parenchymal portions showed weak positive reaction. On the other hand, the suckers, tegument, spines, subtegumental cells, cytoplasm of intestinal epithelial cells, male reproductive organs, and ovary revealed negative staining. The body compartments showing higher antigenicity were, in the decreasing order, the intestinal epithelial border, intestinal content, eggs in the worm capsule, vitelline glands, uterine eggs, and parenchymatous portions.