A cDNA expression library of the larval stage of the cestode worm Echinococcus multilocularis has been established in the phage lambda ZAPII system. By immunoscreening with pooled sera from patients with alveolar echinococcosis an immunoreactive clone, termed pEM13, was isolated. EM13 was expressed using the expression vector pGEX-3X, resulting in the synthesis of a glutathione S-transferase fusion protein. 82% of the sera from 28 patients suffering from E. multilocularis disease had antibodies against EM13, whereas none of the 55 sera obtained from Echinococcus granulosus-infected patients and none of the 15 sera from patients with other helminthic infections reacted with recombinant EM13. By use of a polyclonal rabbit anti-EM13 hyperimmune serum native EM13 protein could be detected only in the protoscolices of E. multilocularis, but not in E. granulosus larvae or hydatid fluid. Immunoelectron microscopy suggested that EM13 is located in the microtriches on the surface of the larvae. In contrast, EM13 mRNA could be detected by Northern blot analysis in both E. multilocularis and E. granulosus larval RNA preparations. Nucleotide and amino acid sequence analysis of a cDNA clone coding for the corresponding antigen of E. granulosus larvae, termed EG13, revealed a 21-bp insertion, a 51-bp deletion and additional 22 nucleotide exchanges resulting in a 96.3% identity at the nucleotide sequence level and a 96.6% identity at the amino acid sequence level to the coding region of the cDNA pEM13. Cross-reactivity of the polyclonal anti-EM13 serum with the recombinant EG13 indicates a posttranscriptional regulation mechanism, resulting in an EG13 negative phenotype in E. granulosus.