The 17 peptides reactive on microarray were carried forward for evaluating their application in ELISA-based serodiagnosis, the current routine technique. Test sera from AE and CE patients and sera from patients with infections other than echinococcosis were used. Sensitivity, cross-reactivity and impact of peptide length were evaluated (data not shown). With respect to sensitivity, long peptides (more than 30 residues) were superior to shorter ones. 3 out of 17 peptides were selected for extensive validation: longD12 (deriving from E. multilocularis EmII/3-10), longD1 (EmII/3-10) and longD8-9 (E. multilocularis antigen B8/1). All 3 peptides did not clearly differentiate between AE and CE infections as can be seen by overlapping standard deviations of A 405nm values obtained from AE and CE sera (figure 1). Peptides longD1 and longD12 derived from EmII/3-10, a well established antigen specific for the diagnosis of E. multilocularis infections [8] . In our hands, longD12 reacted with both, AE and CE sera ( The cut-off values determined from 50 blood donors were similar for longD8-9 and longD1 with 0.165 and 0.200, respectively. The cut-off for longD12 was higher with an OD of 0.357. Compared to 90% sensitivity of a commercial E. granulosus hydatid fluid ELISA used in routine testing [34] , the sensitivity of each single peptide alone was rather low, candidate longD8-9 being the most sensitive with 57%. LongD1 and longD12 both reached 16%. The specificities of longD8-9, longD1 and longD12 were 94%, 91% and 94% respectively, thus performing in the range required for a routine diagnostic assay. There were minor cross-reactivities observed with sera from helminth infections other than echinococcosis (table 3) . Peptide longD8-9 was positive with 1/12 cysticercosis, 1/8 strongyloidiasis and 1/10 schistosomiasis Selection of Diagnostic Antigens by Microarray www.plosntds.org sera. Peptide longD1 was seropositive with 2/8 strongyloidiasis, 1/ 10 toxocariasis, 1/10 filariasis, 1/10 fascioliasis and 3/10 schistosomiasis sera. Peptide longD12 was positive with 1/8 strongyloidiasis, 1/10 toxocariasis, 2/10 schistosomiasis sera. Cross-reactions with Taenia, Strongyloides and filaria are also seen in conventional diagnostic assays. We further tested the specificity with sera from patients suffering from liver abscess due to Entamoeba histolytica infection. LongD12 was positive with 3/26, longD1 with 4/26 and longD8-9 with 3/26 amebiasis sera (table 3) . To compare the diagnostic performance of each peptide, a receiver-operating characteristics (ROC) plot was generated (figure 2). Candidate longD8-9 emerged as clear favorite, although on its own not meeting the sensitivity required for antigens applied in routine diagnostics.