The recombinant Echinococcus multilocularis antigen II/3-10 is one of the most promising tools for immunodiagnosis of alveolar echinococcosis in human patients. Its nucleic acid sequence represents a part of the E. multilocularis gene encoding the metacestode antigen II/3, the former being basically present and expressed in both E. multilocularis and E. granulosus. Most (94%) patients with alveolar echinococcosis respond to infection with a marked anti-II/3-10 IgG synthesis; in contrast, most of the cystic echinococcosis patients do not, for some reason, recognize the recombinant antigen. We tackled this problem by generating cDNA derived from both E. granulosus and E. multilocularis full length II/3 genes, performed by reverse transcription and PCR amplification. Sequence analysis revealed a very high degree of conservation of the primary sequence of the antigen II/3 in both Echinococcus species. cDNA fragments were subcloned and expressed in E. coli as fusion proteins with Schistosoma japonicum glutathione S-transferase. Recombinant proteins were affinity purified and comparatively assessed by ELISA with respect to antibody-binding characteristics. Sera from patients suffering from cystic echinococcosis showed no significant differences in reactivity with the antigens derived from either E. multilocularis or E. granulosus. Therefore, parameters other than some minor differences in the primary sequence seem to be responsible for the lack of antigen II/3 recognition in cystic echinococcosis.