The purpose of this study was to identify species-specific protein components of Echinococcus multilocularis by Western blotting and establish a simple and highly sensitive method for differential serodiagnosis of alveolar hydatid disease (AHD) from cystic hydatid disease. Using Western blotting analyses we demonstrated 2 previously undescribed, species-specific, antigenic components of protoscoleces of E. multilocularis easily detectable with sera from active AHD patients exclusively. The 18 kDa component was recognized by sera from all active AHD patients, whereas the 16 kDa antigen was characteristically detected by patients with advanced lesions. The potential utility of these components for differential serodiagnosis of AHD is discussed.