Fasciola hepatica is responsible for human disease and economic livestock loss on a global scale. Unlike the well characterized schistosomes, only the adult and juvenile stages of F. hepatica are implicated in disease, whereas the freely voided egg is not thought to contribute to host-parasite interactions. We investigated specific immune responses to soluble F. hepatica egg proteins (SFHEP), during a 14-week experimental infection, demonstrating significant increases in anti-SFHEP IgG1 (P = 0 center dot 001), transforming growth factor beta-1 (P = 0.008) and IL-10 (P <0.001) titres at the onset of egg production. Western blot analysis of soluble SFHEP demonstrates that protein bands migrating at 61.6, 54.8 and 44 kDa become sero-reactive before the appearance of eggs within host faeces. Therefore, expression of some egg-associated proteins indicates progression to chronic disease. Antigenic bands were investigated through mass spectrometry, identifying a protein disulphide isomerase (PDI) (61.6 kDa), an enolase and ferritin-related proteins (54.8 kDa), and a cocktail of dehydrogenases (44 kDa). Biochemical analysis of egg secretions reveals proteolytic activity, which increases over time, indicating that proteases may be continually secreted during the course of egg maturation. The implications of egg-specific immune responses and proteolytic secretions are further discussed.