Trichuris suis, the pig whipworm is a nematode parasite located in the large intestine of pigs. Embryonated eggs of T. suis (T. suis ova = TSO) constitute the active pharmaceutical ingredient (API) in a new medicinal product, which is currently tested in human clinical trials as a potential treatment against several immune-mediated disease. In order to ensure that TSO can induce the immunomodulatory effect (pharmaceutical potency) and ultimately the treatment effect, the contained eggs need to be biologically potent. Biological potency refers to the ability of eggs to hatch (measurement of viability) and for larvae to invade the intestinal tissue (measurement of biological infectivity). The current “gold standard” to test the biological potency is by larval establishment in the large intestine of Göttingen minipigs three weeks after inoculation. The minipig infectivity model is reliable, safe, and accurate measure to test the establishment of T. suis larvae (Manuscript 1), where both male and female pigs (3-4 months old) can be used in the assessment of TSO’s biological potency (Manuscript 4). For large-scale, in-process testing of TSO batches where a test result is needed in matter of ho rs, not in weeks, the minipig model is less suited being relatively time consuming and expensive. Thus, faster potency-indicating tests are warranted. One of these alternative potency-indicating tests is an egg hatching assay, which measures the egg viability by enumeration of released larvae from the eggs. The current setup based on mechanical stimulation with glass beads can only to a certain level stimulate eggs to hatch, and the method appears to be more qualitative than quantitative. The success in hatching with mucosal scrapings from the ileum (terminal part of the small intestine) and the large intestine provides basis for further development of a quantitative assay (Manuscript 2). Eggs of the rodent whipworm (Trichuris muris) and of T. suis show biological difference in hatching with selected Gram-positive bacteria and Gram-negative (Escherichia coli) bacteria, which mayreflect their adaptation to their host’s gut environment (Manuscript 3).Short- and long-term storage of unembryonated eggs over a temperature range (5-40°C) has significant influence on the subsequent development of eggs. An accelerated development takes place at 30-34°C, however, higher temperatures suppress embryo formation, and at 40°C the majority of eggs degenerate (Manuscript 4).Overall, the minipig infectivity model offers the most optimal procedure to test the biological potency of TSO. The obtained knowledge from the egg hatching assay allows for further improvements, and if combined with an in vitro migration assay (to assess motility of released larvae) or an in vitro invasion assay (that can mimic intestinal invasion), such in vitro assays may provide an optimal mode-of-action for evaluation of the TSO’s biological potency.