Pentastomids are arthropod-like parasites which inhabit the respiratory tract of tetrapods and, despite being haematophagous, cause little or no visible pathology. Nymphs (or larvae), belonging to the advanced genus Porocephalus, encyst in the tissues of rodent intermediate hosts and migrate from this site to the lung following ingestion of the intermediate host. During migration, several moults occur and instars encounter a variety of tissue barriers and environmental cues. It was therefore relevant to define the proteins and proteinases released by the different instars during this migration as they are likely to be essential for tissue penetration, feeding and immune evasion. In vitro released proteins (IVRP), commonly termed ES, were defined on the basis of molecular size and immune recognition by naturally infected rodent intermediate hosts. In addition, proteinases present in IVRP were characterized using gelatin-substrate gels and protein degradation assays. These studies showed that the protein profile is subtly different when IVRP from different instars is compared. In addition, IVRP from each instar contained metallo-proteinases with proteolysis at 48 kDa dominating in all stages. However, other higher molecular weight activities were, to an extent, released in a stage-specific manner. These proteinases degraded haemoglobin and fibrinogen as well as structural proteins such as collagen and fibronectin, suggesting roles in bloodmeal digestion and tissue penetration. Immunoglobulin digestion was not demonstrated.