Hymenopteran endoparasitoids produce nuclear secretions from ovarian glands, which are deposited into the host insect together with the egg, protecting the developing parasitoid against the host's defence reactions. In the ichneumonid Venturia canescens, virus-like particles (VLPs), are attached to the egg surface and provide passive protection against encapsulation by the host. One of the four major particle proteins (p40) is expressed not only in the calyx gland but also in tissues that are not involved in particle production. The p40 coding DNA from V. canescens was cloned and sequenced. Within the coding DNA a tandem repeat sequence, coding for a putative proteolytic cleavage site of the PEST type, is rearranged in a significant portion of the wasp population. A corresponding polymorphism was also detected in the protein. The amino-terminal region of the deduced protein contains a putative type II transmembrane domain. The carboxy-terminal region shows similarity to the phospholipid hydroxyperoxide glutathione peroxidase (PHGPX) of vertebrates. A peroxidase function of the p40, although not ruled out, is unlikely due to the absence of a reactive centre which is typical for many vertebrate peroxidases. The overall conservation of the hydropathic region is discussed in the context of the formation of the viral envelope and its possible function in the immune protection.