We report the cloning of a gene and the characterization of the encoded protein, which is released by the teratocytes of the parasitoid Aphidius ervi in the haemocoel of the host aphid Acyrthosiphon pisum. The studied protein was identified by LC-MS/MS, and the gathered information used for isolating the full length cDNA. The corresponding gene was made of 3 exons and 2 introns, and was highly expressed in the adult wasps and in parasitized hosts. The translation product, which was named Ae-ENO, showed a very high level of sequence identity with insect enolases. In vivo immunodetection experiments evidenced Ae-ENO localization in round spots, present in the teratocytes and released in the host haemocoel. Moreover, strong immunoreactivity was detected on the surface of A. ervi larvae and of host embryos. Ae-ENO expressed in insect cells was not secreted in the medium, indicating the occurrence in the teratocytes of an unknown pathway for Ae-ENO release. The recombinant protein produced in bacteria under native conditions was a dimer, with evident enolase activity (K(m) = 0.086 +/- 0.017 mM). Enolase is a well known enzyme in cell metabolism, which, however, is associated with a multifunctional role in disease, when present in the extracellular environment, on the surface of prokaryotic and eukaryotic cells. In these cases, the enolase mediates the activation of enzymes involved in the invasion of tissues by pathogens and tumour cells, and in the evasion of host immune response. The possible role played by Ae-ENO in the host regulation process is discussed in the light of this information.