The immunological reactivity of Echinococcus granulosus antigen B was evaluated in 30 hydatid patients. Antigen B was purified from sheep hydatid cyst fluid by electroelution from a non-reducing SDS-PAGE gel (AgB). In ELISA and immunoblotting (IB), determining antibody production in sera from patients with hydatid disease and with other parasitic infections, purified AgB showed higher specificity than a partially purified antigen named pH5PPT (100% vs 83% in pH5PPT-ELISA and 58% in pH5PPT-IB). AgB-IB achieved higher sensitivity than AgB-ELISA (80% vs 63%). All AgB-IB positive sera recognized the 12 kDa subunit. Qualitative AgB-IB assessment of IgG isotype responses identified IgG4 as the predominant isotype (87%). The other isotypes showed a lower percentage of positive reactions: IgG1, 33%; IgG2, 21%; and IgG3, 17%. PBMC proliferative assay revealed a cellular response to AgB in 100% of patients' PBMC. These findings confirm antigen B, especially its smallest subunit, as a good diagnostic molecule.