This work is presented in three Sections, concerned respectively with the in vitro maintenance of Fasciola hepatica, production and properties of the metabolic antigens of P. hepatica, and immunisation of the mammalian host with these antigens. In Section One a continuous-flow culture system for the axenic in vitro maintenance of adult F. hepatica is described. The suitability of various media and culture conditions was assessed by measuring the rate of production of ammonia, an end-product of trematode protein metabolism. The major conclusions were that the addition of serum to the medium was beneficial, Medium 199 was somewhat superior to basal saline and that increased flow-rate of the medium was associated with the output of greater amounts of ammonia. Attempts were made to maintain immature F. hepatica in vitro from the metacercarial stage. Growth and survival were better when the flukes were able to feed on living cells, but compared unfavourably with that expected in vivo. In Section Two it was shown that adult F. hepatica maintained in vitro produced significant quantities of antigenic metabolites. These were shown to contain a variety of components, both heat-stable and heat-labile, which reacted to differing degrees with serum from fluke-infected rabbits, sheep, rats and bovines. Metabolic antigen was used to follow the serological response developing in experimentally infected animals, using double immunodiffusion in agar and the Enzyme Linked Immunosorbent Assay (ELISA). In Section Three a series of experiments was undertaken in which rabbits, rats and mice were immunised with metabolic antigen prepared in various ways from adult or immature F. hepatica maintained in vitro. Although immunised animals consistently developed antibodies to the immunising antigens there was little or no evidence that these had any influence on the course of a subsequent challenge infection.