Abstract: The present invention provides a protective helminth parasite antigen obtainable from adult helminths characterized by: (I) in native form being an integral membrane protein; (ii) having a native localization in the parasite gut; (iii) being capable of binding to a thiol affinity medium; and (iv) being recognized by sera from immunized animal hosts, containing antibodies capable of inhibiting parasite growth and/or development, or a functionally-equivalent variant, or antigenic fragment or precursor thereof, its preparation from adult helminths, its use in vaccine compositions, DNA sequences encoding it and its production by recombinant means.