Antibody and lymphocyte proliferative responses to fluke antigens were compared in rats and cattle following infection with Fasciola hepatica. Antibody responses differed between rats and cattle in that rats responded more quickly and with a greater rate of synthesis over the first three weeks of infection than did cattle. Lymphocyte proliferative responses developed and disappeared at similar times in both cattle and rats, being detectable early in infection, but returning to background levels by week 6 after infection. The presence of antibody but not of lymphocyte proliferative responses correlated with the timing of the development of resistance in cattle and rats as described by others. Sensitisation by intraperitoneal injection of fluke antigens in Freund's incomplete adjuvant (FIA) induced antibody production but not lymphocyte proliferative responses in both rats and cattle. Serum antibodies continued to rise after oral infection of sensitised animals although this was much more marked in the cattle than in the rats. On the other hand lymphocyte proliferative responses were absent when sensitised cattle were infected, while the findings with rats were much more variable. Cattle sensitised by intraperitoneal injection of fluke antigens in FIA were not protected against subsequent infection with F hepatica.