Previous vaccination trials with calves have shown that intramuscular immunization with natively purified activation-associated secreted proteins (ASPs) of Ostertagia ostertagi induces protection against a homologous challenge infection with a 74% reduction in cumulative faecal egg counts and a significant reduction in worm length. Recently, O. ostertagi ASP1 was recombinantly expressed using a baculovirus system and tested in a vaccination trial. However, immunized calves failed to recognize native ASPs and no protection was observed. These results suggest an important structural difference between the baculo r-ASP1 and its native counterpart. Therefore, we investigated whether glycans and/or structural epitopes are key features in the induction of a protective immune response. The results show that ASPs carry two hybrid N-glycosylations with a complex alpha-1,3-arm, an unprocessed alpha-1,6-arm and an alpha-1,6-fucose core. While removal of these glycans had little effect on antibody recognition by vaccinated animals, denaturing and reducing the proteins dramatically reduced recognition, suggesting the importance of conformational protein backbone epitopes.