Cell mediated immunity (CMI) to Fasciola hepatica antigens was detected by lymphocyte proliferation and interleukin-2 (IL-2) production tests in cattle during the first 4 weeks following liver fluke infection. From the fifth week of infection onwards peripheral blood lymphocytes were unresponsive to fluke antigens by these in vitro tests. Investigations into the cause of this unresponsiveness found no evidence to suggest a selective loss of the IL-2 producing lymphocyte sub-population or that macrophages were responsible for the suppression or that antigen responsive cells were being sequestered in the spleen and mesenteric lymph nodes. Tests carried out on culture supernatants demonstrated the production during this unresponsive period of factors capable of suppressing in vitro responses to PHA. Although further tests failed to show antigen specific suppressor factors the presence of MHC restricted suppressor factors could not be ruled out. The early and transient appearance of CMI during F. hepatica infection of cattle indicates that delayed type hypersensitivity is unlikely to be important in protective immunity in cattle.