The pore–forming protein of the human whipworm, Trichuris trichiura, has been postulated to facilitate invasion of the host gut and enable the parasite to maintain its syncytial environment. The data presented here describe the first, to our knowledge, molecular characterization of a pore–forming protein in any helminth and provide a unique demonstration of the functional interaction between a parasite antigen and host molecules. Immunological screening of a T. trichiura cDNA library with T. trichiura infection sera identified a clone of 1.4 kB, the cDNA consisting of 1495 base pairs encoding a protein of 50 kDa. The sequence has a highly repetitive nature containing nine four–disulphide–bonded core domains. Structural prediction analyses reveals an amphipathic nature. TT50 induced pore formation in bilayers in a manner identical to that of the native protein. IgG antibody isolated from T. trichiura infection serum was observed to abolish channel activity.