Abstract: 　Objective To clone, express and serologically evaluate the Em18 antigen gene of Echinococcus multilocularis for diagnostic purpose. Methods Polymerase chain reaction(PCR) was employed for amplification of the target gene fragments which was then ligated with pET28a+ vector. The constructed plasmid was transferred into E.coli BL21(DE3) for expression. The recombinant proteins were purified with Ni\|NTA agarose by affinity chromatography. 237 sera were used for evaluating diagnostic value of the recombinant Em18 antigen. \ Results \ Two high\|level expression clones(designated as ReEm18\|1 and ReEm18\|2) were obtained. ReEm18\|1 showed the expected sequence, ReEm18\|2 showed the same sequence but with 27 nucleotides deletion. The molecular weight of the two expression proteins was \{Mr 28 000\} and \{26 000\}, respectively. Serological evaluation by ELISA was carried out using sera from 101 patients with alveolar echinococcosis(AE), 47 with cystic echinococcosis(CE), 30 with cysticercosis(CC), 10 with hepatic cancer(HC), 9 with schistosomiasis(Sj) and 40 from healthy persons(NH)from both endemic and non\|endemic areas. The results showed an overall sensitivity of 861% and 901% with ReEm18\|1 and ReEm18\|2 for AE sera, specificity 934% and 941%, positive predictive value 906% and 919%, negative predictive value 901% and 928% and efficiency 903% and 924%, respectively. The correlation analysis between the size of AE lesions and the serum absorbance reacted with recombinant Em18 antigens showed that there was a positive correlation between antibody level and the course of the disease. \ Conclusion \ ReEm18 antigens are specific for AE diagnosis, and the serum antibody level displays a good correlation with the course of the disease at early stage. Similar results achieved by both ReEm18-1and ReEm18-2 antigens.

Key words: Echinococcosis, Echinococcus multilocularis, Antigens, Gene cloning, Gene expression