An echinococcus antigen with an apparent molecular weight of 8 kDa was identified as diagnostically important. An immunoblot assay using this antigen was 91% sensitive for surgically confirmed Echinococcus granulosus hydatid disease of the liver. Specificity was 100% for echinococcosis. Marked cross-reactivity was observed with serum specimens from patients with E. multilocularis and E. vogeli infections. The 8 kDa component was not related to the widely recognized echinococcus antigen 5.