In order to provide a better understanding of the interaction between the liver fluke (Fasciola hepatica) and the immune system of its mammalian host immunoreactive λ bacteriophage clones containing F. hepatica cDNA have been isolated. Plasmids from these clones were sequenced and found to encode a family of proteins containing certain common elementsThe nucleotide sequence data reported in this paper are available in the GenBank database with the accession number AF153056.. All the clones contained a coding repeating sequence (RRRXCA) which is conserved at the nucleic acid level followed by a non-repeating element coding for the C terminal used by the proteins which shows conservation of amino acids at certain positions. Antisera raised against a β-galactosidase fusion protein with one of these sequences as a terminal extension was used to localize the immunoreactive antigens. Binding was predominantly in the tegument of the juvenile fluke but was reduced in the adult tegument. The wall of the uterus showed strong reactivity in the adult. Rats immunized with the β- galactosidase fusion protein showed enhanced resistance to challenge infections. The role of these antigens in the host response to infection by F. hepatica is discussed.