In order to observe the localization of the specific antigenic protein in the tissue of Paragonimus westermani metacercaria, immunogoldlabeling method was applied using IgG of the dog which were infected with Paragonimus westermani metacercaria and IgG of rabbits which were immunized with purified 23 kDa protein from metacercaria of the Paragenimus westermani. The metacercaria worm tissues obtained from Cambaroides similis were embedded in Lowicryl HM20 medium, treated with infected and immunized IgG and protein A gold complex (particle size; 12 nm) and observed by electron microscope. In the tissue antigen of Paragonimus westermani metacercaria, the content of excretory bladder which was highly dense electron density was constituted in the excretory bladder of the parenchymal tissue. In the metacercaria tissues antigen reacted with IgG of infected dog. Labeled gold particles distributed on the interstitial matrix of parenchymal cells, fibrous granules of parenchymal tissue and the content of excretory bladder. High antigenicity was observed on content of excretory bladder. It was found to be specifically distributed at the tissue of Paragonimus westermani metacercaria. In the tissues antigen reacted with IgG of immunized rabbit. Labeled gold particles randomly distributed on the interstitial matrix and fibrous granules of parenchymal tissue but in the content of excretory bladder of Paragonimus westermani metacercaria, gold particles were richly labeled. Therefore, the 23 kDa protein contained with Paragonimus westermani metacercaria was found protein which was specifically constituted at the content of excretory bladder of Paragonimum westermani metacercaria. The 23 kDa protein was commonly contained from of Paragonimus westermani metacercaria to adult and showed strong antigenicity against the immunized and infected IgG.