Experiments were conducted to examine ways of monitoring immunological responses in sheep to infection with Fasciola hepatica. The enzyme- linked immunosorbent assay (ELISA), the radioimmunoassay, and the agar gel precipitin technique were evaluated using the same somatic fluke antigen, for sensitivity and accuracy in the serodiagnosis of fascioliasis whilst an in vitro method for culturing lymphocytes of infected and uninfected rabbits and sheep was used to detect an immunological transformational response to the somatic fluke antigen. Comparison was made with haematological changes observed in vivo during the course of an experimental infection. Although ELISA using a tube technique, failed to demonstrate a significant difference between serum from uninfected and infected sheep, a microtitre technique appeared to give satisfactory results. The RIA appeared to provide a highly reproducable and sensitive method for monitoring the antibody response in fascioliasis. Close agreement was obtained between the agar gel precipitin technique using a somatic fluke antigen and RIA. Despite highly variable data and lack of adequate controls, evidence for immunological mediated transformation by sheep lymphocytes was obtained from in vitro cultures made during the prepatent period of a fluke infection. This was correlated to cellular changes seen in vivo. Conversely, there appeared to be an impairment of the cellular response in chronically infected sheep. Thus based on previous authors findings with this technique, evidence suggested that cell mediated immunity existed in fascioliasis. Results obtained using rabbit and sheep lymphocytes showed differences in the cultural kinetics of the different species and indicated the need for further study into the cultural behaviour and requirements of sheep lymphocytes.