A pool of serum samples from mice infected with oncospheres (eggs) of Echinococcus granulosus was used to screen a cDNA library constructed with RNA extracted from protoscolex larvae from sheep hydatid cysts. One immunoreactive clone, designated EpC1, was shown to encode a protein of 76 residues. The complementary DNA (cDNA) fragment was subcloned into an expression vector, pET-41b(+), and the resulting recombinant EpC1 glutathione S-transferase (GST) fusion protein (rEpC1-GST) was expressed in Escherichia coli and was affinity purified against the GST tag. Immunoglobulin G was the dominant antibody isotypes generated against rEpC1-GST. A total of 896 human serum samples were used to evaluate the diagnostic sensitivity and specificity of the fusion protein by immunoglobulin G immunoblotting; 324 serum samples from patients with cystic echinococcosis (CE), 172 from patients with neurocysticercosis, 89 from patients with alveolar echinococcosis, and 241 from patients with other infections or clinical presentations, as well as 70 from confirmed-negative control subjects, yielded an overall sensitivity of 92.2% and an overall specificity of 95.6%. The combined levels of sensitivity and specificity achieved with the rEpC1-GST fusion protein for diagnosis of CE are unprecedented, taking into account the large panel of serum samples that were tested.