So-called "foreign body giant cells" often appear in varied lesions. The foreign body giant cells appearing in inflammatory lesions of five cases were studied by histopathologically, histochemically, and immunohistochemically. As the result, giant cells may be classified into two large groups morphologically: spindle and round type. Judging from the nuclei's disposition and the immunohistochemical result, they seems to be made up by the fusion of many cells of the mononuclear phagocyte system. It is thought that foreign body giant cells change their character due to affecting foreign bodies during the maturational process.